198 BRITISH MOLLUSKS. 



these lias the shell encircled in a very prominent manner by three 

 strongly developed keels, the others are European forms. We have 

 no record of the existence of Valvata in eastern or southern Asia, 

 or in South Africa, Australia, or South America. The genus was 

 supposed, until lately, to be confined to the temperate and north 

 temperate regions of the globe. Valvata appears, however, in its 

 European form, as in the case of Physa and Neritina, in the West 

 Indies. V. depressa, Say, and V. pygmcea, C. B. Adams, from that 

 locality, are almost identical with our V. piscinalis and cristata. It 

 is not improbable that Valvata, or a genus nearly allied to it, exists 

 in other parts of Central America. Mr. Cuming collected several 

 specimens of a shell very like that of Valvata, of a solid keeled 

 type, near Real Llejos, Guatemala, in the bed of a mountain rapid, 

 about four miles from the sea. 



Our two British Valvatce have very different shells. V. piscinalis 

 has a globosely hebciform shell, faintly spirally ridged, while V. cris- 

 tata has a smooth, discoid, Planorbis-like shell. The operculum 

 is in both species concentrically sph'al, rather deeply sutured, and 

 its position is somewhat sunk in the aperture. 



The British species of Valvata are : — 



1. piscinalis. Shell small, of four and a half to five pale, faintly 



ridged, moderately enlarging whorls, convoluted globosely. 



2. cristata. Shell minute, of three and a half to four smooth, 



slowly enlarging whorls, convoluted discoidly. 



1. Valvata piscinalis. Fish-pond Valvata. 



Shell ; somewhat globosely hebciform, deeply narrowly umbibcated, 

 pale straw-colour, semitransparent 

 but solid, whorls four and a half to 

 five, depressed at the apex, longitu- 

 dinally densely very finely striated, 

 spu-ally faintly ridged, ridges some- 

 times obsolete ; aperture somewhat 

 pyriformly rounded. 



Nerita piscinalis and pusilla, Miiller (1774), Yerm. Hist, part ii. p. 171 

 and 172. 



Troclms cristatus, Schroter (1779), Gesch. Fluss-Concli.^. 280. pi. vi. f. 11. 



Helix piscinalis and fascicularis, Ginelin (1788), Si/st. Nat. p. 3627 and 

 3641. 



