PLANORBIS. 



the other Planorbes, although the lower part of the disk is 

 almost flat and carinated at its edge ; and therefore bears 

 a considerable resemblance to the flattened spire of some 

 Land shells, particularly the Helix albella. 



The Genus Planorbis, may be defined as a discoid 

 shell, with a depressed spire, whose apex is always dis- 

 tinct ; its whorls turn from right to left, so that when the 

 spire is held upwards and the aperture seen, it is on the left- 

 hand side ; they are ventricose, frequently carinated, either 

 above or below : the aperture is entire, its breadth equal 

 to its length, sometimes greater, but, we believe, never 

 less; sometimes the peritreine^ or lip, is thickened and ex- 

 panded, and its lower part is always extended forwards : 

 the umbilicus is very much expanded, it has no operculum. 



The recent species are the Helix cornea^ compianata, 

 carinata^ Spirorbis^ Vortex of Linn.^ and such other fresh 

 water shells as agree with them in general form, and bein^ 

 destitute of an operculum ; these are common European 

 shells : one or two species are found in America, with 

 thickened and expanded lips. 



Several fossil species abound in the distinctly fresh 

 W'ater strata of the Isle of Wight and the neighbourhood 

 of Paris, where they are very abundant, and accompanied 

 by as great a profusion of Limnei and some other decidedly 

 fresh water shells ; it is worthy of remark, that one of the 

 commonest at the Isle of Wight, has a blunt carina at its 

 lower edge, and its umbilicus so flat and extended, as to 

 cause the lower part to be easily mistaken for the spire, but 

 it may be distinguished by the center on the flat side being 

 umbilicate, and the apex being seen on the other side. 



Fig. 1. Planorhis corneus. — Lam. 



2. Guadaloupensis n. 



3. carinatus. — Drap. 



4. ■ hicarinatus n. 



5. Euomphalus. — Sotaerhy, 



Of these, the first four are recent species ; P. corneus and 

 jP. carinatus^ being common English species ; P. bicarinatus 

 is an American species, with an expanded and thickened lip ; 

 we have named it from its being carinated both on the side 

 of the spire and on that of the umbilicus. The Planorbis 

 Euomphalus is the fossil species, of which we have spoken 

 more particularly above. 



Some species, particularly when young, are cdvcred 

 with an hairy epidermis. 



