PLANOKBIS. 
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 rhomhea 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 rliombea of Tur- 
ton was probably a Scalaris monstrosity of H erice- 
torum, {Per. 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. 
