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. Planoebis coeneus. Linn. Tab. I, fig. 12, a — c. 



Helix cornea. Linn. Syst. Nat. p. 1243, 17G6. 



— Mont. Test. Brit. p. 448, 1803. 

 Planorbis corneus. 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. 1 1, 1837. 



— 5. Wood. Catalogue 1842. 



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

 anfractibus teretibus rapide crescentibus ; aperturd lunato rotunda. 



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

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

 impressed by the previous whorl. 



Diameter, § 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 finespiral strice, and the volutions, in more advanced 

 age, are sometimes angulated. The strias 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 strias. 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 depres- 

 sion of the vertex appears rather an anomaly in the mode of volution, but it occa- 

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

 of the Bulla, where the lip is elevated above the spire, giving it there a deep umbi- 

 licus. 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 pro- 

 tective covering to the animal. 



