266 PNEUMONOBRANCHIATA. 



Planorbis deformis. Lam. vi. 1 54. ? 



Var. 3. entirely without any keel. Alder, 1. c. 113. 



Monstrosity, with the volutions elevated into a 



spiral cone. 



Helix terebra. Turton, Diet. p. 62. f. 55. 

 cochlea. Brown, Wern. Trans, ii. t. 24. f. 10. 



In stagnant waters and slow rivers. 



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 distinc- 

 ness and prominence, but is never so prominent as in 

 the former species. 



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. Bot. ii. 113.) 



Ferussac thought that the Helix rhombea of Turton 

 was probably a Scalaris monstrosity of H. ericetorum. 

 (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. 



