FAMILY CARDIACEA. 227 



sometimes tumidly obtuse, ligament subexternal, now conspicu- 

 ous, now barely discernible. Hinge composed of a small 

 double cardinal tooth, and two elevated lateral teeth in the 

 left valve, interlocking with another double tooth and four 

 lateral teeth of the same character, in the right valve. 

 The Pisidia, or Pea Cockles, differ essentially from the Cyclads, 

 with which they were formerly confused, both as regard the animal 

 and its shell. Not only do the siphonic tubes differ, as represented 

 in our respective vignettes, the tubes in Pisidium being txnited in 

 one, while the tubes in Cyclas are united for a little distance and 

 then separated, but the shells differ also. It will be seen, that in 

 Pisidium, the anterior side of the shell, the side from which the 

 foot is protruded, is the longer, whilst in Cyclas, the posterior side, 

 the side from which the siphonic tubes are exserted, is the longer. 

 This important distinction, which appears to have escaped the 

 notice of Mr. Jeffreys, is well defined by Forbes and Hanley, and 

 by Baudon. The ligament may be detected, by a careful observer, 

 to be on the shorter side of the shell in Pisidium, on the longer 

 side in Cyclas. The valves of the shell in Pisidium are more 

 depressed than in Cyclas, and the species are uniformly smaller, 

 though each genus has a comparatively large species of its own, 

 P. amnicum and C. rivicola, each larger than any representative in 

 any other geographical province. 



The European Pisidia, of which some thirty to forty species have 

 been described, are probably reducible to seven or eight, of which 

 we have five in Britain, occiirring in all three kingdoms. They are 

 not, however, so generally diffused on the Continent. P. amnicum, 

 nitidum, and obtusale are found throughout Europe, but P. amnicum 

 is the only species that ranges into Siberia and North Africa, while 

 P. obtusale is confined chiefly to the central parts. P. pusillum 

 ranges into Siberia, and is not found in the south of Europe. 

 P. Casertanum and pulcliellum are found in the south, and not in 

 the north. The animal of Pisidium is extremely delicate, trans- 

 parent, and almost gelatinous. 



The foreign range of the genus is rather scattered, and apparently 

 very imperfectly known. In addition to about twelve European 

 species of Pisidium, there are records in Mr. Cuming's collection of 

 the presence of a species of rather large size in Lycia, collected at 

 an elevation of 6500 feet, one in India, P. parvula, Benson, col- 

 lected by Dr. Bacon, and one in New Zealand, not yet described, 

 collected by Mr. Strange. 



