VALVATA. 75 



outer edges of which are membranous and project in front so 

 as to make slight ridges. L. 0-025. B. 0*125. 



Habitat : Lakes, canals, pouds and ditches, with the 

 last species. It has an equally extensive range both 

 here and abroad, except that the present species does 

 not appear to have been found south of Corsica. It is 

 also one of our upper tertiary fossils. A monstrosity is 

 in my collection, which has the last whorl detached and 

 coiled, upwards, as is not unfrequently the case with 

 several species of PIano7'bis. 



The brancbial plume is not always protruded, even 

 when the animal is crawling. 



Although the spire in some specimens is a trifle more 

 depressed, or sunk than usual, I am not aware that the 

 Valvata spirorbis of Draparnaud (which Moquin-Tandon 

 regards as a variety of the present species) has ever been 

 found in this country. The V. minuta of that author is 

 a totally different species, having a globular shell, with a 

 produced spire, and resembling a miniature V.piscinalis. 

 The present species is the V. planorbis of Draparnaud. 



There is no difficulty in distinguishing V. cristata 

 from the fry of V. piscinalis, much less from the adult, 

 where the great difference of size affords a sufficient 

 criterion. The shell of the present species in all stages 

 of growth is quite flat, and resembles that of a Planorbis, 

 constituting apparently a passage into that genus ; while 

 the other is trochoid or subglobose, and. has a prominent 

 spire. Their bodies, or the soft parts of the animal, do not 

 present such a decided difference. The tentacles of V. 

 piscinalis are, however, rather more slender, and the 

 snout is proportionably larger than in this species. 



e2 



