62 



The shell of this species is of an ashy horn colour, 

 often covered with an outer brown epidermis, rather 

 shining and nearly smooth. It is about half-aii-inch 

 in diameter with five deeply-divided whorls, which 

 gradually incline to the outer edge, which is more or 

 less keeled. Upper disk rather concave, lower disk 

 flat. 



var. DISCIFORMIS, has the lower disk almost convex, 

 and the keel more central. 



Hob. Stagnant or slowly-running waters in almost 

 every district, but it is local. More common in the 

 south than the north. Very fine at Norton, near 

 Stockton. 



P. MARGINATUS. Drapcirnaud. PI. VII, fig. 8. 



Shell brown colour, slightly concave above, flat beneath, striolate, 

 not polished, carinated. 



Helix planorbis, Linn. 



This species very much resembles the last, but may 

 be distinguished from it by being thicker and having 

 the whorls more rounded, the outer one being more or 

 less carinated near the base, the upper part of which is 

 rounded, .not shelving. The mouth is rhombic, but 

 rounded in front. It is covered with an olivaceous 

 black, or ferruginous coating, which is not polished, and 

 distinctly wrinkled. About half-aii-inch in diameter. 



The shells included under this species vary much 

 both in shape and colour ; the keel, which in some 

 specimens is prominent, is scarcely discernible or alto- 

 gether wanting in others. It has, by some naturalists, 

 been divided into several distinct species. 



var. RHOMBEUS, shell higher than usual, with the 

 gyrations rather more compact, more convex above and 

 concave below ; keel almost obsolete. 



var. DRAPARNALDI, somewhat smaller than the 

 above, and resembles a young carinatus. 



Hob. This is by far the commonest of the Planorbes, 



