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Significant differences remain between gastropod phylogenetic hypotheses based on morphological and molecular datasets. We collected additional data from three gene segments (28S rDNA expansion region D1 (36 taxa plus two from GenBank), cytochrome c oxidase subunit 1 (35 species plus one from Genbank) and small nuclear RNA U2 (24 species)). These were combined with data available for the same species for histone H3 and two other segments of 28S rDNA. Analyses of these data using cladistic, maximum likelihood or Bayesian methodologies were conducted in an attempt to resolve some of the differences between current hypotheses of gastropod relationships based on morphological and molecular data. The results were of particular interest in four areas. (1) Patellogastropoda in most analyses are included in a derived clade with some Vetigastropoda. In an analysis with Nautilus as the sole outgroup, transversions weighted threefold as costly as transitions and, with third codon position data ignored, Patellogastropoda are excluded from an otherwise monophyletic Gastropoda. (2) Cocculiniformia was never monophyletic in our analyses, although this possibility is not statistically rejected. (3) Nerita, the only representative of Neritopsina in this dataset, is placed anomalously in most analyses, but is, in a few cases, shown as a sister-group to the Apogastropoda, in accord with some morphological hypotheses. (4) Heterobranchia is rarely monophyletic in our analyses owing to the variable placement of the architectonicoid Philippea. This genus, even judged by the high levels of divergence within Heterobranchia, has undergone extreme rates of substitution. The Euthyneura is invariably monophyletic and nearly always included in a clade with the valvatoidean Cornirostra as its sister-group.

K-Ar ages (A. Abdel-Monem, P. D. Watkins, and P. W. Gast, 1971, American Journal of Science 271, 490–521; this paper) and revised paleontological determinations (J. Meco, 1977, “Los Strombus neogenos y cuatenarios del Atlantico euroafricano”, Las Palmas, Ediciones del Excmo. Cabildo Insular de Gran Canaria) show that “Quaternary” (R. Crofts, 1967, Quaternaria 9, 247–260; G. Lecointre, K. J. Tinkler, and G. Richards, 1967, Academy of Natural Science of Philadelphia Proceedings 119, 325–344) littoral deposits on Lanzarote and Fuerteventura are early Pliocene and late Pleistocene. Early and middle Pleistocene strand lines are not represented. Early Pliocene littoral and marine deposits contain a characteristic fossil assemblage: Strombus coronatus, Nerità emiliana, Gryphaea virleti, Patella cf. intermedia, and Rothpletzia rudista. Differences in elevation record differential post-Pliocene uplift of the coastal platforms on which they lie. Late Pleistocene beach deposits at low elevations belong to two groups, an older with Strombus bubonius and a younger without. Differences in elevation of early Pliocene littoral deposits are reflected by differences in elevation of late Pleistocene beach deposits nearby.

Populations of the intertidal hermit crab Calcinus tibicen were observed in the laboratory and reproductive behaviors recorded. Of the 218 interactions, 68 resulted in copulation(s). Male and female sizes were positively correlated. Male size affected copulation success in a non-linear fashion. In particular, the largest males did not obtain any copulations. This was largely a consequence of the shell species occupied by large individuals; males in Nerita sp and Cittarium pica shells were unsuccessful in courtship. The ability to execute precopulatory rotation of the female was negatively affected by certain shell types. Repeated pairings of individuals suggested some level of individual recognition within the reproductively active population.

The patterns of dispersion of Nerita atramentosa, Bembicium nanum and Cellana tramoserica within two major substrata, the encrusting alga Peyssonelia gunniana and bare rock were analysed from quadrat samples taken at low tide, over 1 year. Distances to the nearest members of each species from random points were analysed to determine spatial patterns of dispersion. The frequencies of nearest neighbours of each species to each animal in the sampled area were analysed to determine the degree of spatial segregation and symmetry in the three species. Nerita atramentosa and Bembicium nanum showed significant aggregated (clumped) dispersion in both substrata. Cellana tramoserica, however, was dispersed at random on Peyssonelia, and regularly dispersed on bare rock. There was no correlation between index of dispersion and density, or time of year, in any species. No useful tests for segregation and symmetry between Cellana and the other two species could be made because of the difference in dispersion of this species. Nerita and Bembicium were significantly segregated on both substrata, indicating that the microhabitat requirements of the two species are different. The two species were symmetrical on both substrata, indicating that there is no tendency for members of one species to be more isolated than members of the other species. Possible explanations of the differences in spatial pattern shown by the three species are discussed, and experimentally testable hypotheses are proposed to clarify the importance of physical and biological factors in the spatial patterns of distribution of the three species.

Intrinsic rhythmic activity of Nerita (Melanerita) atramentosa melanotragus was assessed under constant laboratory conditions. Activity proved to be a sensitive indicator of toxicity, and was affected by low levels of a relatively new oil dispersing agent, Shell SD LTX. How the state of activity influenced animal sensitivity to Shell SD LTX and to an oil, Maui Condensate, was investigated using short-term recovery experiments. Nerita were most sensitive during their active phase, and results during this period differed significantly from tests carried out during the inactive phase of the animal. Dispersant/oil mixture proved to be highly toxic. These findings have ecological implications and permit comment relating to the design of sublethal toxicity tests. These subjects are discussed.

Populations of three coexisting intertidal gastropods, Nerita atramentosa Reeve, Bembicium nanum (Lamarck) and Cellana tramoserica (Sowerby), were sampled from a shore in Botany Bay, New South Wales, from July 1972 to September 1973. The recruitment and growth rates of each species were analysed from size frequency distributions. Mortality of each age cohort, and longevity, were estimated from analyses of the densities of Nerita atramentosa and Bembicium nanum . Nerita atramentosa showed no significant mortality during the first two years on the shore, but high mortality (at an instantaneous rate of 0.084 deaths/individual/month) after reaching the age of reproductive maturity, which was 20 months from settlement and at a mean shell-length of 13.5 mm. No growth could be detected after reproductive maturity was reached. Longevity of N. atramentosa was estimated as 3–5.5 years. Bembicium nanum juveniles showed higher mortality (at an instantaneous rate of 0.233 deaths/individual/month) than reproductively mature animals (0.060 deaths/individual/month). Reproductive maturity was reached at a mean shell-breadth of 11.0mm, i . e ., about ten months after settlement on the shore. Longevity was estimated as from 4–8 years. Cellana tramoserica showed different growth patterns depending on the time of settlement. The three species showed different patterns of growth and life history relating to variable recruitment (which was demonstrated in all three species) and different rates of mortality of age cohorts.

Oligocene-Miocene rocks of the Torquay Basin contain at least 55 different species of calcareous nannofossils. These species allow zonation of an offshore subsurface section (Nerita No. 1 Well) and two onshore sections (near Torquay). The Angahook Formation and the base of the Jan Juc Formation in the Nerita No. 1 Well belong to Martini's (1971) NP 22 calcareous nannofossil Zone (early Oligocene). The NP 23 Zone could not be identified with certainty, but Zones NP 24/25 (late Oligocene) to NN 2 (early Miocene) were recognized in the Jan Juc Formation. The Oligocene-Miocene boundary is present between 122 m and 135 m below the sea floor at this site. The Puebla Formation, exposed at the sea floor, contains NN 5 spanning the early-middle Miocene boundary. Onshore along the Torquay coast, NP 24/25 (late Oligocene) is present in the Angahook Formation; the Jan Juc Formation at Bird Rock also contains NP 24/25, except the uppermost 2.5 m which belongs to NN 1. The NN 2 Zone begins at the contact between the Ja...

The archaeogastropod Nerita scabricosta is an abundant and extremely mobile herbivore which occurs from above mean high water through the mid—intertidal zone of exposed rocky areas along the Pacific coast of Panama. It is the principal prey of Purpura pansa, a patchily distributed, high intertidal neogastropod. Both snails exhibit activity patterns and habitat preference which appear to have evolved in response to heavy predation pressure by fish and to harsh physical factors. These behavioral mechanisms restrict movement and limit foraging time for both species, but also result in a cyclic overlap of their distributions. Potential interaction between the two gastropods is thus limited to well defined, discrete periods. Purpura elicits both an escape and an avoidance response in Nerita. The intensity of predation on the nerite by Purpura is lessened by these responses and Purpura appears unable directly to control the abundance of Nerita. However, it is experimentally shown that in a heterogeneous environment, Nerita's distribution is still more restricted and its local abundance further reduced by avoidance responses to temporally stable aggregations of Purpura. These aggregations, centered on specific fish—safe microhabitat, are thus hypothesized to result in the creation of local patches of intertidal substrate which are free from herbivory or disturbance effects by Nerita scabricosta. Evidence from field observations and manipulations is presented to support the hypothesis that in this complex, four trophic level community, a predator—prey interaction between two gastropods can limit potential resource monopolization and affect primary space occupancy.

The highly variable brackish-water snail Clithon oualaniensis (Lesson1831) was first studied in Hong Kong, Ceylon and Malaya-Singapore. Populations within these regions are closely similar, but there are consistent differences between regions. Subsequently, particularly round the Malay Peninsula, orderly changes of variants have come to light. This raises the question of how these have come into being and what is responsible for their apparent stability. Contrary to first impressions, most of the variability of Clithon and of certain species of Umbonium and Nerita is pseudo-polymorphic in nature. Conventional polymorphism seems to make but a small contribution. Quasi-continuous variation and pseudo-polymorphism are essentially the same phenomenon. Both arise by the interaction between polygenic inheritance and environmental conditions. Whether the resulting pheno­types are continuous or discontinuous depends on the developmental physiology of the organs or structures involved and is thus a secondary phenomenon.

In southwestern Baja California Norte a new species of Early Tertiary Tethyan gastropod, Velates, V. vizcainoensis n. sp., occurs with a previously unknown intertidal molluscan fauna suggestive of an unprotected coastline of probable Paleocene age. Comparison of Velates vizcainoensis n. sp. to other forms previously assigned to Velates from southwestern North America suggests that “Nerita” cuneata Gabb of Late Cretaceous age probably belongs in Neritina (Dostia), as do N. (D.) aff. N. (D.) cuneata and N. (D.) escondita n. sp.Velates is probably derived from typical Otostoma and is an early Tertiary genus. Velates californicus Vokes, although originally described as being from the Llajas Formation of middle Eocene age, is from the upper Santa Susana Formation and probably is of late Paleocene age at its type locality.Two other neritid gastropods are present in the fauna with Velates vizcainoensis; namely, Corsania (Januncia) Janus n. subgen., n. sp. and Nerita (Theliostyla) n. sp.(?). Corsania Vidal, 1917, is a genus distinct from Otostoma and ranges from Early Cretaceous to Paleocene. Januncia n. subgen. has a Tethyan distribution and ranges from Maastrichtian through Paleocene.

The oxygen consumption of three species of Malaysian mangrove gastropods was measured in air and sea water at the temperatures commonly recorded in the mangrove. The experiments in air were carried out after the animals had regained fluid lost from the mantle due to handling. Fluid loss can have considerable effects on rate of oxygen consumption. Nerita arliculata (Gould) was found throughout the mangrove and experiences from 50 to 92% aerial exposure. It has a gill and a ratio of aerial to aquatic respiration rates of 2.7 at 28°C. 50% of the animals can survive underwater for 72 h at 28°C. The other two species, Cerithidea obtusa (Lamarck) and Cassidula aurisfelis (Brugiere) experience over 95% aerial exposure, have their mantle cavities modified as lungs and have air : water respiration rate ratios of 5.5 and 6.0, respectively. 50% can survive from 48 to 36 h underwater at 28°C. Acclimated animals have Q10's of about 1.6 in air and 1.4 in water. The respiratory physiology of the snails is compared with that of rocky shore species.

Field observations and experiments were conducted on the intertidal gastropod Nerita textilis Gmelin along the Somalian coast to determine if its rhythmical mass-homing includes the detection of durable substrate marking as well as short-term trail-following. The snails' first response to displacement is a zonal orientation compensating for the vertical component of experimental shifting. The homing performance of symmetrically transferred animals supports the hypothesis that a marked area is present in and below the aggregation site, detectable by the homer snails 24 hours after its deposition by spontaneously moving conspecifics. No specific marking of different collective homes resulted from these experiments.

Intraspecific competition for food within and between age-classes of the intertidal grazing gastropod Nerita atramentosa Reeve was investigated using experimental cages on a rock platform in Botany Bay, New South Wales. Adults (mean length 21.0 mm) and juveniles (mean length 12.36 mm) were confined separately and together, at densities of 10, 20 and 50 per cage. The lowest density was the same as the mean density of Nerita on the shore. Cages were maintained for 146 days from November, 1973 to April, 1974. Each animal was numbered and its growth rate recorded over the period of the experiment. Dead animals were replaced by others of the same size to maintain densities, but replacements were not included in analyses. Proportional mortality of each size-class in each cage, growth rates and wet tissue weights at the end of the experiment were analysed. Adult animals did not grow. Growth rate of juveniles decreased with increasing density. Mortality of juveniles was not significantly different at different densities, but mortality of adults increased linearly as density increased. Wet tissue weights of adults and juveniles decreased with increasing density. In adults there was a decrease in weight at densities of 20 and 50 per cage. In juveniles, tissue weight increased with growth, from the start of the experiment except at densities of 50/cage where tissue weight remained constant and growth was absent. For all three factors, there were no differences between the effects of increasing density by additional members of the same age-class or by members of the other age-class. Adults and juveniles have, therefore, identical effects on the growth rates of juveniles, mortality of adults and wet tissue weights of both age-classes. Population density regulation in Nerita is possible, since high densities (e.g., due to increased juvenile recruitment) will cause reduced growth of juveniles and increased mortality of adults, until the densities are lowered. This effect of increased density is compared with intraspecific competition reported in other intertidal gastropods.

Field observations have been made of short-term acute and long-term sublethal responses of Nerita (Melanerita) atramentosa melanotragus , to experimental applications of Maui Condensate, and a relatively new oil dispersing agent, Shell SD LTX. Short-term responses suggested condensate/dispersant mixtures to be highly toxic, whereas weathered condensate and LTX, when applied separately, produced no significant mortality. In the longer term, however, sublethal effects such as changes in wet weight, gonad weight, and gonad fertility indices were especially noticeable on LTX treatments. Unexpectedly large reductions in weight, and aberrations in gonad tissue structure were recorded. Ecological implications of these results are discussed.

The densities of populations of Nerita atramentosa Reeve, Austrocochlea constricta Lamarck, Bembiciuin nanum (Lamarck), and Cellana tramoserica (Sowerby) were recorded in 0.25 m2 quadrats on landward and seaward halves of a sheltered and a moderately exposed rock platform from June 1972 to June 1973. The data have been subjected to variance and multiple regression analysis. Differences between densities of each species on the two shores and in the two areas of each shore are discussed with respect to the regressions on littoral height. Densities of Nerita on the sheltered shore, Austrocochlea and Bembicium on both shores, and Cellana, on the exposed shore, increased with height. The density of Cellana on the sheltered shore decreased as height increased. The density differences in the landward and seaward areas on the two shores were attributable to the effect of height for Nerita and Austrocochlea. Height was the major variable affecting the density of these two species on the sheltered shore. Bembicium and Cellana densities on both shores, and Austrocochlea density on the exposed shore, are not primarily determined by height. On the sheltered shore, Nerita and Cellana increased in density with increasing cover of water in rock pools. On the exposed shore, Bembicium and Cellana became sparser with increasing water cover. Bembicium on both shores increased in density with increasing cover of the alga Peyssonelia, as did Cellana on the exposed shore. Cellana and Austrocochlea tended to increase in density with Nerita on the sheltered shore. Cellana tended to be sparse where Bembicium was dense on the exposed shore. The biological effects of substrata and other species on the densities of each species remain unexplained. Significant time effects were found for variations in the densities of Nerita and Cellana on the sheltered shore and of Austrocochlea and Cellana on the exposed shore. The seasonal change in density of Nerita was parabolic with a minimum in summer, and is consistent with the known period of recruitment of this species. The lack of significant time effects on density of Austrocochlea on the sheltered shore is consistent with the known continuous recruitment of this species. The declining density of Austrocochlea on the moderately exposed shore was found, by size-frequency analysis, to be due to reduced recruitment compared with surrounding shores. The density of Cellana on both shores showed seasonal changes, with a maximum in summer, which is consistent with the known period of recruitment in Cellana. The lack of seasonal changes in density of Bembicium is briefly discussed. This type of multifactor analysis identifies biological interactions affecting variation in density of these species. It also corroborates the results of other ecological investigations and determines the relative importance of a variety of physical and biological factors in the distribution of density of intertidal gastropods.

The reproductive behavior of two species of diogenid hermit crabs was studied in Hawaii. In the shell generalist, Clibanarius zebra, male reproductive success varied little with size, although the largest males were less successful in obtaining copulations than were medium-large males. Male and female size were positively correlated, in successful pairs, thus larger males had the potential to fertilize more eggs when they were successful in obtaining a copulation. Female fecundity in C. zebra was not affected by species of gastropod shell inhabited once female size was taken into account. Male copulatory success was very strongly influenced by the species of gastropod shell inhabited. Males in Trochus or Nerita shells had greatly reduced reproductive success compared to males in Turbo or Nassarius shells. This result was due both to (1) males in Trochus especially dropping and otherwise poorly handling females during precopulatory behavior and (2) females not responding to precopulatory behavior patterns executed by males in Trochus and Nerita. Transferring males from “good” to “bad” shapes of shells and vice versa showed that male success was a function of shell type inhabited and not some correlated feature of the crabs. In the shell specialist, Calcinus seurati, which is found primarily in Nerita shells as an adult, males in Nerita shells were quite successful in obtaining copulations.

The rates of growth of two common intertidal gastropods, Nerita atramentosa Reeve and Bembicium nanum (Lamarck), were measured during ≈ 3 months in cages at four heights on a shore at Cape Banks, New South Wales. Replicate cages at each level were used to keep snails at constant densities throughout the experiments. Amounts of food on the substratum in each cage were measured as the concentration of chlorophyll in samples of the rock, which is known to be a reliable estimator of number of algal cells present. Rates of feeding were measured as the number of radular scrapes per min in snails removed to the laboratory, and the sizes of radulae were measured at the end of the experiments. Nerita did not increase in shell-length at the two higher levels on the shore, but grew at the two lower levels. Rate of growth was positively correlated with the amount of chlorophyll in the substratum, and was not apparently affected by the greater period of submersion (and concomitant time available for feeding) towards the lower levels on the shore. There was no difference in rates of feeding at the four heights. Radulae of snails from the lower levels were longer than those from higher on the shore, but this was attributable to the greater overall size of snails from lower levels (i.e. there was no significant difference in relative size of radula from one height on the shore to another). Bembicium increased in width of shell at all levels on the shore, but grew more quickly at all three lower levels than at the top of the shore. There was no difference in rate of feeding, nor in relative size of the radula from one level to another on the shore. Rate of growth was not significantly correlated with the amount of food available, nor with the period of submersion, nor the product of these two variables. The simplest explanation for these results was that sufficient food was available for maximal growth of Bembicium at the three lower levels, but not at the highest level. Thus, only where the amount of food was inadequate, at the highest level, was rate of growth reduced. These results are discussed with reference to the difficulties of determining the relative importance of influences on the growth of intertidal snails, because of inherent confounding of the period of submersion with different amounts of food at different heights on the shore.

This widely-spread, and, on the whole, well-marked species, has been in so many cases, as it appears to me, confounded with Alaria bispinosa, that it would be a task of no small difficulty to trace out the mistakes that have arisen in consequence. The imperfect figures of Phillips have, no doubt, been the principal cause of thus confounding the two species. Deslongchamps queried the identification, but yet was inclined to adopt it. Morris and Lycett seem to have had no doubt on the subject. “Having had the advantage of examining a large number of specimens, comprising every variety both in form and stage of growth, we feel no hesitation in uniting the two species here indicated.” It is extremely probable that the Great Oolite specimens of Alaria trifida present varietal differences from the Oxfordian types, and they may also in some cases have assumed forms having only one lateral digitation; but higher up in the Jurassic rocks they seem to be very fairly separable for such a genus as Alaria, which is so prone to effect changes in its shell. Such was doubtless the opinion of Morris, who quotes Al. bispinosa from the Lower Calc. Grit and Kel. Rock, whilst Al. trifida is quoted as a distinct species from the Oxford Clay and Kel. Rock of Yorkshire, the Great Oolite form being referred to a variety.

Population energy budgets estimated on the assumption of steady state conditions for Nerita tessellataGmelin, N. versicolorGmelin, and N. peloronta L. on Barbados, W. Indies, are presented. Large differences in population structure, and hence energetics, occurred at different localities along the beach. Relatively high proportions (81 to 88%) of the assimilated energy were lost via metabolism. Assimilation efficiencies ranged from 39 to 43%, net growth efficiencies from 5 to 13%, and ecological efficiencies from 3 to 7%. For each species, production (P), energy flow (A) and total energy consumption (C) were expressed as functions of animal size, in order to facilitate gross estimations of the energy components for other populations for which data on size-frequency and density are available. Respiration studies of all three species in the laboratory failed to detect differences between respiration rates in air or under seawater.

The complex reproductive systems of the mangrove-dwelling Nerita birmanica are described and related to those of other neritids. Gonad condition in both sexes and numbers of spermatophores in males and females varied slightly during a one-year survey. It is concluded that a slight tendency for larger number of “full” ovaries with big oocytes to occur in collections taken shortly before full and new moon does not represent a significant lunar cycle of gonad activity since almost all gonads bore gametes in all stages of development at all times of year. It appears merely that ovulation may be held up during relatively rainless periods after which substantial rain is followed by reduced ovaries lacking large oocytes. In brief, breeding appears to be continous with but slight variation which does not consistently follow observable environmental factors.

The little capsules in which snails of this family deposit eggs are so durable as sometimes to be found adherent to museum specimens; hitherto, however, detailed descriptions of such capsules have been restricted to the capsules long known as made by Neritina fluviatilis in fresh waters of Europe. The present paper adds descriptions, with measurements and sketches, of Neritina reclivata from Florida, Neritina virginea, Neritina punctulata, Theodoxus mneleagris, Nerita peloronta, Nerita alticola, Nerita tesselata and Neritilia succinea from Jamaica, British West Indies. These capsules are found to be remarkable in that the digestive tract aids in their formation; that is, the capsules are made as secretions of the lower oviduct to which mineral particles are added, and these mineral particles are supplied from the intestine and stored up in a special sac opening into the oviduct. Moreover, the mineral particles are of twofold origin; in some Neritinas they come from the ooze taken in with the food and passed through the intestine; in some Neritas they come from the liver as calcospherites, to be passed through the intestine. It is hoped that the data given will aid future descriptions of capsules from other species till a firm foundation is built for utilization of these capsules as aids in taxonomy.

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The shell selection behaviour of Coenobita scaevola Forskal has been examined in a natural population by statistical methods and by selection experiments using shells whose weights had been artificially changed. In a natural population of C. scaevola there was a definite weight index (weightof the shell/weight of the animal) for the shells of each species of snail. The weight index for one population ranged between 0.7 and 13.0, with a maximum (46% of all values) between 1.3 and 1.9. The volume index (volume of the shell/weight of the animal) ranged from 0.7 to 4.0. The maximum (59%) for the shells of all snail species lay between 1.0–1.3. Small specimens of C. scaevola tolerate greater differences in the weight and volume indexes of their host shells than large specimens. A comparison between weight index and volume index shows that shell selection depends on volume. This is probably also true for a small hermit crab, Clibanarius signatus Heller, of the tidal zone. Selection experiments on the weight index showed maximum value for Galeodes of about 1, for Turbo at about 2, and for Nerita between 2 and 4. Selection experiments on the volume index showed that, independent of the species of the occupied snail shell, there was a maximum between 1.0 and 1.5. In selection experiments between shells of different snail species but of equal volumes the selection was made in regard to their weight indexes. At a weight index lower than 1.0 only Natica was preferred, between indexes 1.0 and 1.5 Natica and Galeodes were similar, while at a weight index of 2.0 or more Turbo was chosen. The shells of those snail species which are generally used by Coenobita scaevola are found in the same frequency in the snail fauna of a place as they are occupied by C. scaevola. With the exception of a few species of aquatic hermit crabs which are characterized by flat thoraxes and which live in shells with a long and narrow opening, all other water and land hermit crabs seem to have no relation to the shells of a given snail species. C. scaevola e.g., lives in the shells of 29 snail species. Broken shells or shells with a fouling of calcareous algae are better tolerated by water hermit crabs than by land hermit crabs. Land hermit crabs also live in fossil and subfossil shells.

Nerita undata, N. plicata, N. polita, N. albicilla and N. textilis are common intertidal gastropods on Aldabra Atoll. Each species prefers a different level on the shore and/or different degrees of exposure to wave action. Patterns of zonation, population size-frequency structure and biomass are given. N. undata exhibits 2 distinct ecophenotypes. Normal types occur on expsed to moderately sheltered shores but are replaced by ‘type B’ in extremely sheltered conditions. Differences in shell morphology and population structure are also noted. Foraging by all species was confined to hours of darkness, generally at low tide. Activity of N. polita was induced slightly on overcast days. N. undata and N. textilis are cryptically coloured and often visible during the day. N. plicata is conspicuous, but is protected by a strong shell. N. polita and N. albicilla are often brightly and variably coloured, but both are concealed during daylight. No Nerita were found infected with trematodes, suggesting that the degree of infection was extremely low. Habitats of the Aldabran Nerita are compared with 3 species on Barbados, West Indies.

The reduced right half of the pallial complex of Nerita tessellata, N. fulgurans, N. versicolor, N. peloronta and Neritina virginea comprises a hypobranchial gland, a vestigial gill and the opening of a duct which bears the same relationship to this gill as that of the left kidney to the functional gill. The duct leads from the region of the genital duct corresponding to the renal section of mesogastropods. The posterior part of the glandular genital duct lies in the visceral mass; its relationships are identical with those of the left kidney. The anterior part develops alongside the rectum within the anterior pallial vein. The left half of the pallial complex of Nerita includes a hypobranchial gland absent in Neritina and Theodoxus. The floor of the mantle cavity of Nerita forms an accessory respiratory surface, receiving blood from the anterior aorta. Its blood spaces communicate with the haemocoel and the parabranchial vein. This vein, also present in Neritina and Theodoxus, drains the mantle skirt. In Nerita its importance increases when the snail is out of water and the mantle cavity is used as a lung. The buccal mass of Nerita fulgurans has the same basic organization as that of Monodonta lineata: differences are related to specializations in the neritid radula requiring greater versatility of movement during feeding. It is protruded by lateral and ventral protractor and buccal depressor muscles originating on the body wall and inserted on the posterior radular cartilages. The horns of the anterior cartilages are orientated to the feeding ground by an array of levator muscles unparalleled in the trochid, and by the buccal fold which also spreads the radula and ensures its maximal sweep. The radular membrane is pulled outward by protractor muscles originating on the posterior cartilages and associated with a cartilage in the posterior wall of the sublingual pouch. Food is gathered on the back stroke of the radula which is drawn into the buccal cavity by a powerful retractor muscle associated with a radular diverticulum. The diverticulum is absent in trochids which have muscles for direct retraction of the buccal mass. The absence of salivary glands in neritaceans may be related to the development of the buccal fold; glandular buccal pouches and sublingual glands may compensate for their loss. The oesophageal glands extend through the anterior oesophagus to the buccal cavity; owing to their separation from the mid-oesophagus they are unaffected by torsion. The stomach is a voluminous sac, simplified in that the caecum is lost or vestigial and that the major typhosole ends at the junction of the style sac with the proximal region. The gastric shield forms a tooth, very prominent in Neritina, which can protect the openings of the oesophagus and ducts of the digestive gland.

Previous investigations have shown that competition for space among sessile organisms in rocky intertidal communities is often reduced by predation or harsh environmental factors. Grazing gastropods are unlikely to compete for space, but at high densities might compete for food, unless some factor, such as predation, reduced the densities. The coexisting species of herbivourous gastropods on rock-platforms in New South Wales are not, however, subject to high levels of predation on shores sheltered from waveaction. In this study, three of these species were caged at densities from the natural, to 4 times the natural density on the shore, in different combinations of species, to investigate their competitive interactions. All three species, Nerita atramentosa, Bembicium nanum and the limpet Cellana tramoserica, showed increased mortality and reduced weight at increased density, over 100- or 200-day periods. The effect of high density on Cellana was greater than on Nerita or on Bembicium. In addition, Nerita was competitively superior to the other two species. Cellana, at high densities, adversely affected Bembicium. Nerita was not affected by high densities of either of the other two species, and Cellana was not affected by Bembicium.Under entirely natural conditions, the effects of increased density of Cellana on the mortality and tissue-weight of Bembicium could not occur, because of the high rate of mortality of Cellana when at increased density of its own species. The other effects of increased density of snails would, however, occur. Thus, there can be regulation of numbers of snails because of high densities of their own or other species on the shore.The supply of benthic, microalgal food is proposed as the limiting resource for which the species compete. Hypotheses are proposed to account for the mechanisms by which the three species acquire different amounts of the resource. These are based on aspects of the feeding biology of the snails.The continued coexistence, in intertidal regions, of Cellana, which shows the highest rate of mortality due to members of its own species, with the competitively dominant Nerita, which increases the mortality of Cellana, is apparently due to three factors. These are: (i) the presence of subtidal refuges for breeding populations of Cellana, (ii) regular spatial dispersion of Cellana which would probably decrease intraspecific competition for food, and (iii) the high variability in space and time of recruitment of planktonic larvae of both Cellana and Nerita. This last factor makes it unlikely that high densities of Nerita could occur on all shores in every year. Cellana could always recolonize any area where its density had previously been reduced, and thus, cannot be excluded by competitive interactions. Competitive interactions at high densities of gastropods can therefore cause reductions in the number of each species present on a shore. They cannot, however, lead to exclusion of any species or alter the composition of the community of herbivores on the shore.The difference between competition for space by sessile intertidal organisms, and competition for food by gastropods which graze on microalgae is discussed with respect to the structure of intertidal communities. In the former case, competitively dominant species must be dislodged from the resource, space, by disturbance (e.g. predation or hazards) before recolonization by inferior competitors can occur. In the latter situation, the food resource is renewable without further perturbation of the community, and the competitively dominant species could never consume it completely over a wide enough area, and for a sufficient length of time, to eliminate totally another species.

Field experiments were used to determine the effect of a common intertidal snail (Nerita funiculata) on the use of space for foraging by the hermit crab Clibanarius digueti. Removals of Nerita resulted in an increased density of foraging Clibanarius, while additions of the gastropod had the opposite effect. The observed negative effect of the gastropod on individual hermit crabs appears to be food-related. Field surveys, however, suggested that the hermit crab population is limited by shell number, rather than food. Because Nerita contributes to the shell resource, its effect on the hermit crab population is positive. Nerita, therefore, has a negative effect on the distribution of foraging hermit crabs, but a positive effect on their abundance. Such decouplings of distribution and abundance effects are rare.

The patterns of dispersion of Nerita atramentosa, Austrocochlea constricta, Bembicium nanum and Cellana tramoserica are analysed from quadrat samples taken between June 1972 and June 1973. In each quadrat, the proportion of animals in each of two substrata, the encrusting alga Peyssonelia gunniana and small pools, was recorded, with the proportional area of cover of each substratum, height on the shore, density of the species and the time of sampling. Multiple regression and covariance analysis showed that, in all cases, there was a linear relationship between proportion in a substratum and proportional area of substratum. No other variables were significant. Juvenile Nerita and Austrocochlea showed preferences for both substrata. Adult Nerita and all Bembicium were dispersed at random with respect to both substrata. Adult Austrocochlea were randomly dispersed with respect to Peyssonelia, but showed a preference for water. Cellana showed a preference for water and against Peyssonelia. These microhabitat preferences are discussed in relation to previously described effects of substrata on the densities of these four species.

Nerita albicilla [spec. nov.] N. testa striata, labiis subdentatis: interiore tuberculato. Rumph. mus. t. 22. f. 8. Habitat ad Hitoe., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 778, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

Nerita albume [spec. nov.] N. testa umbilicata convexa, umbilico subcordato: lobo explanato. List. conch. 4. s. 5. c. 3. f. 1, 2. Rumph. mus. t. 22. f. B. Vitellus compressus. Gvalt. test. t. 67. f. A, B. Argenv. conch. t. 10. f. T. Kratzenst. Regenf. 20. t. 5. f. 54. Habitat in O. Asiae., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 776, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

Nerita vitellus [spec. nov.] N. testa umbilicata subglobosa, umbilico perforato aequali. Rumph. mus. t. 22. f. D. Valvata 5. Habitat in O. Asiae., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 776, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

Nerita lacustris [spec. nov.] N. testa laeviuscula cornea, apice exquisito, labiis edentulis. Habitat in Europae lacubus. Testa similis N. littorali, sed cornea, nec vertice carioso. * * * Imperforatae labio dentato., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 778, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

The purpose of the following brief notice is to put on record some observations made in the island of Skye in August 1850, by which the true geological horizon of the so-called “Wealden” of Loch Staffin was determined, and the Oxford Clay added to the series of oolitic strata in the Hebrides. Sir Roderick Murchison, in his “Supplementary Remarks on the Strata of the Oolitic Series and the Rocks associated with them in the Counties of Sutherland and Ross, and in the Hebrides,” read before the Geological Society in November 1827, states that “in the low and ruinous cliff of blue shale, associated with zeolitic and amygdaloidal trap on the north-eastern shores of Loch Staffin, were found, during my late excursion with Professor Sedgwick, flattened masses of shelly limestone containing five species of Cyclas ) one Paludina , one Neritina , one Ostrea , one Mytilus , and some undescribed bivalves,” and remarks that “it adds materially to the interest of these remains, that two species of the Cyclas , the Paludina , and the Ostrea prove to be identical with the fossils of one of the upper beds of the Weald clay described by Dr. Fitton as occurring in Swanage Bay, Dorsetshire, and in the Isle of Wight.” Of these fossils a list is appended to the paper, drawn up by Mr. Sowerby; and besides the references to Weald Clay species, one Cyclas is considered identical with a Barton Cliff shell, and the Nerita is compared with a Woolwich species. When the Duke of Argyll announced

Nerita plicata [spec. nov.] N. testa sulcata, labiis dentatis: interiore rotundato; exteriore utrinque dentibus acutis conicis. M. L. U. Habitat.., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 779, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

Nerita polita [spec. nov.] N. testa laevi, labiis utrisque dentatis. Rumph. mus. t. 22. f. I, 2. Valvata alpina. Gvalt. test. t. 66. f. C. Argenv. conch. t. 10. f. K. Kratzenst. Regenf. 16. t. 4. f. 43. Habitat in O. Asiae., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 778, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

Nerita fluviatilis [spec. nov.] N. testa rugosa, labiis edentulis. Fn. svec. 1318. List. angl. 136. t. 2. f. 20. conch. 2. p. 1. t. 38. Pet. mus. 67. n. 718. Habitat in Europae cataractis., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 777, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

Nerita histrio [spec. nov.] N. testa sulcata transverseque striata, labio interiore dentato. Rumph. mus. t. 22. f. 6. Habitat.., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 778, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

Nerita grossa [spec. nov.] N. testa sulcata, labiis dentatis: interiore convexo rugoso. Rumph. mus. t. 22. f. N. Gvalt. test. t. 66. f. V. Habitat in O. Asiae., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 779, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

Nerita bidens [spec. nov.] N. testa laevi, labio interiore bidentato. M. L. U. Habitat.., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 778, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

Nerita radula [spec. nov.] N. testa sulcata tuberculis aequalibus, labiis edentulis. M. L. U. Habitat.., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 777, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

Nerita mammilla [spec. nov.] N. testa umbilicata ovata glabra, umbilico obtecto, apertura ovata. Column. aqv. t. 52. f. ult. Cochlea nivea. List. conch. 4. s. 5. c. 3. t. 1. f. 3, 4. Rumph. mus. t. 22. f. E. Valvata 7: Albula. Gvalt. test. t. 67. f. C, D. Argenv. conch. t. 10. f. X. Uber tenue. Habitat ad Barbados. * * Imperforatae labio edentulo., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on pages 776-777, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

Nerita nodosa [spec. nov.] N. testa nodosa pluribus ordinibus solida, labro angulato. Bonan. recr. 3. t. 173, 174. List. conch. t. 804. f. 13. Gvalt. test. t. 66. f. BB. Klein. ostr. 17. t. 1. f. 30. Habitat.. Affinis Murici Nodo tota structura, sed postice lacuna destituta, qua neglecta ruerent genera., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 777, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

Nerita exuvia [spec. nov.] N. testa sulcata, labiis dentatis: interiore tuberculato. Bonan. recr. 3. t. 220. List. conch. 4. s. 1. c. 1. t. 2. f. 5. Rumph. mus. t. 22. f. M. Pet. gaz t. 100. f. 6. Gvalt. test. t. 66. f. CC. Argenv. conch. t. 10. f. I. Habitat in O. Americae & Asiae., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 779, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

Nerita glaucina [spec. nov.] N. testa umbilicata laevi, spira obtusiuscula, umbilico semiclauso: lobo gibbo dicolore. List. conch. 4. s. 5. c. 1. t. 5. f. 1. Rumph. mus. t. 22. f. A. Valvata laevis. Gvalt. test. t. 67. f. M, P, T. Habitat in O. Europaeo., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 776, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

Nerita canrena [spec. nov.] N. testa umbilicata laevi, spira submucronata, umbilico gibbo bifido. Rumph. mus. t. 22. f. C. Valvata 4. Gvalt. test. t. 67. f. E, Q, R, S, V, X. Argenv. conch t. 10. f. C. Kratzenst. Regenf. 10. t. 3. f. 34. Habitat in O. Asiae., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 776, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

Nerita [gen. nov.] Animal Limax. Testa univalvis, spiralis, gibba, subtus planiuscula. Apertura semiorbicularis: labio columellae transverso, truncato, planiusculo. * Umbilicatae., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 776, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206

Nerita undata [spec. nov.] N. testa sulcata sulcis triginta, labiis dentatis: interiore rugoso tuberculato. Rumph. mus. t. 22. f. 4. Gvalt. test. t. 66. f. X. Habitat in O. Asiae ad Amboinam. Differt a praecedenti etiam apice magis prominente, & fasciis latiusculis, nec tenuibus striis., Published as part of Linnaeus, Carolus, 1758, Systema Naturae per regna tria naturae: secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Stockholm :Laurentius Salvius on page 779, DOI: 10.5962/bhl.title.542, http://zenodo.org/record/3922206