226 BRITISH MOLLUSKS. 



pared with. Cyclas, is in the proportion of one to two, whilst in the 

 Western Hemisphere it is as two to one. We have in Europe 

 about twelve species of Pisidium and six of Cyclas ; in the United 

 States there are about six species of Pisidium and twelve of Cyclas. 

 Nearly twice as many species are recorded by a recent mono- 

 grapher of the two genera, Mr. Temple Prime, but a careful ex- 

 amination of many of them in Mr. Cuming's collection leads me 

 to think that probably balf will have to be suppressed. Both 

 genera appear in ]N"ew Zealand and Central America, and a species 

 of Pisidium has been described by Mr. Benson, from India. 

 They are probably much more generally diffused abroad than we 

 are at present aware of, although they are partially represented in 

 the streams of warm, temperate, and intertropical countries by three 

 important genera of bivalves of large size unknown in Europe — 

 Cyrena, Velorita, and Galathcea. 

 The British freshwater Cardiacea belong to two genera : — 



1. Pisidium. Animal having the siphons united in one tube. 



Shell thin, horny, slopingly produced in front. 



2. Cyclas. Animal having the siphons united in one tube for 



some distance and then separated. Shell thin, horny, slo- 

 pingly produced behind. 



Pisidium amnicum. (Slightly enlarged.) 



Genus I. PISIDIUM, C. Pfeiffer. 



Animal ; body oval, greyish, delicately transparent, mantle lobes 

 united at each end, foot large, broad, attenuately elongated, 

 siphonal tube sometimes short and obliquely subcorneal, some- 

 times elongately conical. 



Shell ; equivalve, inequilateral, anterior side the longer, more or 

 less slopingly produced, thin, covered with an obve-horny 

 epidermis, valves mostly concentrically ridged, or striated, 

 interiorly bluish- white, umboes sometimes rather prominent, 



