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This study describes 18 specimens representing the earliest growth parts of orthoceratoid cephalopods, most of which have been recently obtained from the Middle Ordovician strata of the Prague Basin, central Bohemia. Only about 20 specimens of Ordovician orthoceratoids displaying embryonic shell are known globally and the Bohemian material thus significantly expands their current record. The studied collection includes twelve specimens assigned to the species Bactroceras sandbergeri and shows that the morphological variability in early ontogenetic growth stages of this species was rather low. The absence of a cicatrix was definitely confirmed in this phylogenetically important cephalopod. Six other specimens investigated herein are indeterminate orthoceratids. Initial chambers, where preserved, are generally large compared to known, stratigraphically younger specimens. This is interpreted here as supporting the previously postulated trend of decreasing the egg- and hatchling sizes in orthoceratoid cephalopods from mid-Ordovician times onwards. Alternatively, the large sizes of these Bohemian orthoceratids may be explained by the position of the Prague Basin in relatively cool-water high palaeolatitudes during the Middle Ordovician. [ABSTRACT FROM AUTHOR]

A cephalopod Peismoceras Hyatt, 1884 from the Silurian (Gorstian) of Bohemia is the single known nautiloid genus of the suborder Barrandeocerina with a documented colour pattern. However, spiral bands on lateral side of the shell, oblique to the shell axis, illustrated by J. Barrande in 1865 in Peismoceras pulchrum and later mentioned in several papers are not typical for this species. Zigzag bands are more common and they are developed either on the entire circumference of the shell or on one lateral side, while the other side retained the above mentioned spiral bands. A marked asymmetry of colour pattern on lateral sides has been observed in several cases, but with no proven relationship to the slight dextral coiling of the shell. Densely spaced longitudinal bands preserved on ventral side of P. asperum (Barrande, 1865) are illustrated for the first time. Some unusual shell abnormalities occurring in Peismoceras are discussed. [ABSTRACT FROM AUTHOR]