186 BRITISH MOLLUSKS. 



BytTnnia tentaculata. 



Genus I. BYTHINIA, Gray. 



Animal; elongated, subcylindrical, carrying a light turbinated 

 shell, head produced into a lengthened muzzle, cleft in front, 

 tentacles slenderly elongated and flexible, with the eyes sessile 

 at their outer base, foot oblong-triangular, broad in front, at- 

 tenuated behind, bearing a subtestaceous operculum, which is 

 sometimes paucispiral, with a lateral nucleus, sometimes con- 

 centric, with the nucleus in the centre. 

 Shell; conically turbinated, minutely umbilicated, fulvous green, 

 semitransparent, smooth, of five to seven more or less rounded 

 whorls, covered with a slight horny epidermis, aperture pyri- 

 formly ovate, with the margins continuous. 

 Before treating of the large and well-known Paludince of our 

 canals and rivers, we have to speak of three much smaller rnollusks 

 formerly included in the same genus, until separated on very suffi- 

 cient grounds by Dr. Gray, under the title of Bythinia. The Palu- 

 dince bring forth their young alive, and the tentacles are cylindrical 

 and firm, with the eyes raised upon stalks. The Bythinice are ovi- 

 parous, and the tentacles are long and slender, almost filiform, with 

 the eyes sessile ; the operculum differs also in being superficially 

 testaceous. In B. tentaculata and LeacJiii, the operculum is formed 

 of concentric additions round a central nucleus, but in B. similis, 

 a species associated on the banks of the Thames with Assiminea 

 Grayana, the operculum is spiral, and two-whorled, with a lateral 

 nucleus like that of its associate. Mr. Jeffreys includes it in a 

 separate genus Hydrobia, along with the marine Rissoa ventrosa 

 of authors. M. Moquin-Tandon refers it, along with nine other 

 species, natives of the brooks and rivers of France, to the genus 

 Bythinia, but as a separate section, Bythinella. The shell of 



