GASTEROPODA. 123 



the volution, becoming nearer together towards the lower part, and terminating with 

 one that is elevated into a sort of keel, encircling the umbihcus, which is broad and 

 naked, but not very deep. 



Trochus,* Linn. 

 Pyramis. Schmn. 1817. 

 ZiziPHixus. Leach, 1819. 



GlBBCLA. Id. 



Steromphala. Id. 



Pykamidea. Swainson, 1840. 

 Calliostoma. Id. 



Gen. Char. Shell conical, vA\h. a more or less elevated spire, thick, strong, and 

 of a nacreous texture ; apex generally acute, sometimes depressed and obtuse ; base 

 flattish, with a carinated edge to the volutions ; aperture square or rhomboidal, with 

 an oblique columella, parallel to the outer lip ; operculum corneous and spiral. 



This is a marine genus, and not restricted to any climate, being found in all parts 

 of the world. It is also of great antiquity ; it appears first in the Protozoic rocks, 

 Mr. G. B. Sowerby, in his genera, has separated Trochus from Turbo by the dis- 

 tinguishing character of a corneous operciolum for the former, and a testaceous one for 

 the latter ; these two genera merge so imperceptibly into each other, that they are not 

 to be defined by the form of the shell alone, although the species of this genus are 

 generally more truly conical. 



Sect. a. IMPERFORATE. 



1 . Trochus crenularis. Lam. Tab. XIII, fig. 7, a — b. 



Tkochus crenclakis. Desk. Coq. foss. des Env. de Par. p. 229, pi. 27, fig. 3, and pi. 28, fig. 13, 15. 

 — Bowdich. Elements of Conch, pi. 9, fig. 7, 1822. 



Tr. Testa elongato-conicd, pyramidatd, elevatd, imperforatd ; transversim hiberculatd ; 

 anfractimm margine inferiore crasso, tuberculis majoribus crenato ; cohimelld tmncatd, 

 incrasssatd. 



Shell pyramidal or elongato-conical, imperforate ; sides rather flat, ornamented 

 with transverse tuberculatcd striae, inferior one most prominent ; base flat ; aperture 

 trapezifonn, with a truncated columella, thickened, or unidentated at the lower part. 



Axis, 1 inch. 



Locality. Cor. Crag, Gedgrave. 



The specimen from which the figure was taken is the only one I have seen ; it was 

 obligingly sent to me by Mr. Charles worth for description : it is from the cabinet of 

 H. Davey, Esq., of Beccles, and w-as purchased, among other Crag fossils, at Orford, in 

 Sufi'olk. This specimen so precisely resembles some of my Grignon shells, that it is 

 requisite to record the manner it has come into my possession, as there is a slight 



* Etym. Trochus, a top. 



