10 MOLLUSCA FROM THE CRAG. 



This is figured and described from a single specimen, belonging to Mr. Lyell. Its 

 outer edge is rounded and not carinated, corresponding in that respect with the recent 

 shell, which is now considered by most conchologists to be distinct from P. vortex, 

 in which one edge is strongly keeled, and the surface thereby made quite flat, with the 

 suture reduced to a fine and narrow line. Both these species are found in the fresh- 

 water deposit, at Clacton, although P. vortex is the most abundant. 



3. PLANORBIS CORNEUS. Linn. Tab. I, fig. 12, a c. 

 HELIX CORNEA. Linn, Syst. Nat. p. 1243, 1766. 

 Mont. Test. Brit. p. 448, 1803. 

 PLANOEBIS COENEUS. Drap. Moll. pi. 1, fig. 42, 44, 1805. 



Gray. 2d edit. Turton's Man. fig. 95, 1840. 

 G. B. Sowerby. Genera, fig. 1, 18? 



Lam. 2d edit. Hist. Nat. des An. sans Vert. viii. p. 382, 1837. 

 Rossmasler. Icon. t. 2, p. 14, pi. 7, fig. 113, 1838. 

 Dujardin. Mem. Soc. Geol. de France, t. 11, 1837. 



S. Wood. Catalogue 1842. 



PL Testa discoided, tenui,fragili, plano-depressa, altero late et profunde umbilicata ; 

 anfractibm teretibus rapide crescentibus ; aperturd lunato rotunda. 



Shell discoidal, thin, and fragile, rather flat on one side, with a deep or canaliculated 

 suture ; umbilicus broad and deep ; volutions subcylindrical, rapidly increasing, 

 impressed by the previous whorl. 



Diameter, f of an inch. 



Locality. Mam. Crag, Bulcham. Recent, Britain. 



The figure is from a specimen in the cabinet of Capt. Alexander, and the only one 

 I have seen. It corresponds precisely with specimens of the same size, of the common 

 recent British species, and there is no doubt of its identity. The recent shell, in its young 

 state, is marked with fine spiral striae, and the volutions, in more advanced age, 

 are sometimes angulated. The striae are more especially distinct in the perios- 

 traca, which, when removed, leave the shell nearly smooth. Our specimen has the 

 slightest possible trace of spiral striae. This species has much the character of a 

 sinistral shell, taking that for the under side in which the spire is most depressed 

 and umbilicated ; but the portion of the peritreme is more produced and projecting on 

 that side which, if it were a dextral shell, would be the uppermost. The depression 

 of the vertex appears rather an anomaly in the mode of volution, but it occasionally 

 occurs in species that are convolute upon a horizontal axis, as among some of the 

 Bullte, where the lip is elevated above the spire, giving it there a deep umbilicus. 

 Moreover, I should imagine that the projection of the lower portion of the aperture of 

 the shell would rather impede the action of the foot, and incommode the motions of 

 the animal. I have therefore considered that to be the upper side (fig. 12, b), in 

 which the projecting portion of the peritreme would act rather as a protective covering 

 to the animal. 



