50 PTEROPODA. 



thin texture, transversely striated or smooth, sometimes with 

 marginal ribs, but without lateral appendages. The mouth 

 of the shell is trigonal, and in some forms, at any rate, is 

 furnished with an operculum (tig. 442, e), or occasionally 

 furnished with curved lateral appendages. The length of 

 the shell varies, but is commonly from an inch to an inch 

 and a half. The genus is apparently allied to the recent 

 Clio, but the dimensions of the shell much exceed those of 

 any known species of the latter. Barrande enumerates 

 eighty-four Paheozoic species, principally distributed in the 

 Upper Cambrian and Silurian, but occuriing also in the De- 

 vonian and Permian. Ptcrotlwca (fig. 442, A and b), of the 

 Silurian, in many respects resembles Hijolitkcs ; but the 

 median dagger-shajDed shell is bordered by lateral concentri- 

 cally-striated expansions or alations, thus coming to super- 

 ficially resemble the carapace of certain of the Phyllopods. 



The Silurian genus Colcoprion has a cylindrical and conical 

 shell, the exterior of which is marked with chevron-shaped 

 strife. Possibly allied to the preceding tyj)es, but of very 

 uncertain affinities, are the genera Hemiceras and Salter ella. 

 In the first of these are conical elongated shells, of circular 

 section, in which the walls are thickened by the deposition 

 of concentric calcareous lamelhe, till only a small tuljular 

 space is left in the centre. Tlie genus is Silurian. Sal- 

 terclla, of the Upper Cambrian, comprises conical tubes, 

 resembling the preceding in shape, but consisting of several 

 hollow cones placed one within the other. 



Tlie genus Coiiularia is one of the most extraordinary of 

 the extinct genera of the Pteropods, if only for the enormous 

 size attained by many examples. The shape of the shell is 

 very like that of some living Pteropods, but specimens occa- 

 sionally reach the length of nearly a foot, with a breadth of 

 more than an inch. The shell in Conidaria (fig. 443) is 

 straight, tapering towards one end, and having a sub-quadrate 

 or rhomboidal aperture at the other. Tlie form of the shell is 

 generally distinctly four-sided, the sides being finely striated 

 with transverse lines. The shell is generally of extreme 

 tenuity ; but the internal cavity is sometimes restricted by 

 concentric lanielhie, and the ajiex may be partitioned oft'. 



