PLANORBIS. 241 



Shell about three quarters of an inch in diameter, 

 very like the last, but thicker and the whorls more 

 rounded, more convex to the edge beneath, and flat- 

 ter at top or behind ; hence the keel has been called 

 marginal, and the mouth is more rhombic and rounded 

 in front ; these characters are quite as visible in the 

 young shells. The keel greatly varies in distinct- 

 ness and prominence, but is never so prominent as in 

 the former species. In some examples it is continued 

 along the edge of the penult turn, in others it is 

 almost altogether obsolete. 



These animals breed very rapidly in ponds of 

 the warm water that is emitted from steam engines 

 in Yorkshire ; but the specimens which are found in 

 such situations have a great inclination to assume 

 the regular spiral form, with a deep umbilicus. It 

 is to be observed that these shells are all dextral. 



There is no doubt but that the Helix rhombea of 

 Turton is only the young state of this species ; and 

 Dr. Leach's specimen of Planorbis Sheppardi, which 

 is the type of Dr. Turton's P. complanatus, is evi- 

 dently the same ; his figure is half as large again 

 as the specimens in the Museum. Mr. Sheppard 

 thought it was allied to P. albus ; and this, perhaps, 

 misled Mr. Alder to think that it might be a variety 

 of that species. (Mag. Zool. and Bot. ii. 113.) 



Ferussac thought that the Helix rhombea of Tur- 

 ton was probably a Scalaris monstrosity of H. erice- 

 torum. (Fer. Prod.) Dr. Fleming considered that 

 the Helix terebra of Turton might be a distortion 

 of Helix lapicida; but Dr. Turton has reduced it 

 to this species. 



R 



