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Tucetona isabellae Valentich-Scott & Garfinkle, new species Figures 1 A ���G Glycymeris (Glycymeris) cabazoni Bramkamp, 1935, unnumbered pages, plate 2, figures 2, 3; Tucetona (Bellaxinaea) new species, Powell, 1986: 79 ���81, plate 2, figures 5, 7. Shell shape. Subovate to subtrigonal, moderately inflated, height and length about equal; beaks narrow, pointed, opisthogyrate. Sculpture and color. About 29 (20���48, n= 15) heavy, broad radial ribs, many bifurcate in larger specimens, overlain by strong, fine, well-spaced, commarginal ribs forming crossbars over ribs and in interspaces; interspaces moderately wide, frequently with intercalary ribs; periostracum inconspicuous; exterior color tan, with few dark brown blotches; interior color white to cream, with brown mottling in some specimens, some specimens mostly brown internally. Hinge. Hinge plate curved, narrow, posterior tooth series about 8 (4���11, n= 10), anterior about 7 (3���11), ligament asymmetrical, much longer anteriorly, moderate in length, moderately narrow, with about 3 (2���6) chevron grooves. Adductor muscle and pallial scars. Moderate in size, moderately impressed, anterior scar subovate, posterior scar ovate-elongate; pallial line narrow, with long, narrow dorsally directed lines extending from it. Inner ventral crenulations. About 24 (14���43) rectangular, flat topped. Distribution. Modern specimens are only known from a small region off the northwest end of Isla Smith, Baja California, Mexico (29.1�� N), 120��� 170 m. Fossil specimens are present in the late Miocene ���Imperial��� Formation in Riverside County, southern California (Powell, 1986; 1988). Type locality and type specimens. Mexico, Baja California, off the northwest end of Isla Smith; 29 ��05��� 12 ���N, 113 �� 32 ��� 12 ���W; 120��� 170 m. All paratypes listed below are from the same lot as the holotype. Holotype. SBMNH 149636, length 14.5 mm, height 15.0 mm. Paratypes. SBMNH 149637, 34 paired valves, 31 separate valves; BMNH 20100629, 2 paired valves, 2 separate valves; CASIZ 184502, 3 paired valves; USNM (1149245), 2 paired valves, 2 separate valves. Etymology. Named in honor of Isabella M. A. Rocha from Santa Barbara, California, a close friend of the junior author. Comparisons. Tucetona isabellae differs from Tucetona bicolor Reeve, 1843 (Figures 1 H���J), which has a subtrigonal shell shape, about 39 radial ribs with moderately shallow, very narrow interspaces, with very fine, very closely spaced commarginal ribs, and a moderately wide, curved hinge plate. Tucetona multicostata G. B. S ow e r b y I, 1833 (Figures 1 K���M) has a subovate shell shape, about 32 non-bifurcate radial ribs with wide, deep interspaces, with moderately spaced commarginal ribs, and a moderately wide, curved hinge plate. Tucetona isabellae differs from Tucetona strigilata G. B. Sowerby I, 1833 (Figures 1 N���P), which has a subtrigonal shell shape, about 24 non-bifurcate radial ribs with moderately wide, moderately shallow interspaces, with very fine, closely spaced commarginal ribs, and a wide hinge plate with curved teeth. The North Atlantic Tucetona pectinata (Gmelin, 1791) has fewer, non-bifurcating radial ribs with narrow interspaces when compared to T. isabellae. The new species has a similar number of radial ribs when compared to the northern Atlantic Tucetona subtilis Nicol, 1956, but the ribs are not bifurcate, and the latter species has fewer teeth on the hinge plate. Remarks. Tucetona isabellae has also been found in the fossil record although never formally described (Bramkamp, 1935; Powell, 1986). Powell (1986) compares this species to two fossil species from Venezuela and Columbia, both of which have significant differences in dentition and ligamental grooves, and thus are not included in the comparisons above. We have done a close examination of the Panamic members of the genus Tucetona and found that it has been difficult to differentiate the species. Many of the specimens in the museum collections we examined were incorrectly identified due to a misunderstanding of important characters within the genus. We found that the most distinct differences between the different Panamic species were, 1) presence or absence of rib bifurcation, 2) width and depth of interspaces, 3) size of the commarginal ribs, and 4) the width of the hinge plate. These characters alone can still sometimes lead to incorrect identifications. Further study, using molecular tools, likely would produce additional characters and might lead to the discovery of additional new species., Published as part of Valentich-Scott, Paul & Garfinkle, Elizabeth A. R., 2011, A new species of Tucetona (Bivalvia: Glycymerididae) from Mexico, pp. 65-68 in Zootaxa 2769 on pages 65-67, DOI: 10.5281/zenodo.276860, {"references":["Bramkamp, R. A. (1935) Stratigraphy and molluscan fauna of the Imperial Formation of San Gorgonio Pass, California. Unpublished Ph. D. dissertation, University of California, Berkeley, 86 pp. + unnumbered pages, 10 pl.","Powell, C. L., II (1986) Stratigraphy and bivalve molluscan paleontology of the Neogene Imperial Formation in Riverside County, California. Unpublished M. S. thesis, San Jose State University, San Jose, California, 324 pp., 13 pl.","Powell, C. L., II (1988) The Miocene and Pliocene Imperial Formation of southern California and its molluscan fauna: an overview. Western Society of Malacologists Annual Report, 20, 11 - 18.","Reeve, L. A. (1843) Monograph of the genus Pectunculus. Conchologia iconica; or, illustrations of the shells of molluscous animals, 1, 9 pls.","Sowerby, G. B., I (1833) [... the collection of shells formed by Mr. Cuming on the western coast of South America, and among the islands of the southern Pacific Ocean]. Proceedings of Zoological Society of London, 1832, 194 - 202.","Gmelin, J. F. (1791) Caroli a Linne ... Systema naturae per regna tria naturae ... editio decima tertia, acuta, reformata, Lepzig (Beer), 1 (6), 3021 - 3910.","Nicol, D. (1956) Distribution of living glycymerids with a new species from Bermuda. The Nautilus, 70, 48 - 53."]}