124 CYCLADID^. 



longer anterior side has its extremity tapering, and almost 

 symmetrically rounded ; the posterior termination is broad- 

 ly and very bluntly rounded, but is subangulated above 

 from the little curvature of the very short and scarcely de- 

 clining hinder dorsal edge. The ligament is scarcely per- 

 ceptible ; the umbones are broad, and but little projecting. 



From the valves becoming more quickly shallow (the 

 profundity is chiefly confined to the umbonal region) than 

 in ohtusale, they meet below at an acute angle. There 

 exists a variety in which the umbones are less blunt and 

 more prominent ; and another shining one, on which the 

 striulas are more distinctly graven. The extreme length 

 is a line and three-quarters, the breadth is nearly a line 

 and a half. 



The animal, according to Jenyns, is white, with a short 

 entire margined siphonal tube, which varies its shape from 

 cylindrical to subconic. The foot is a little longer than the 

 length of the shell. 



This is apparently by far the commonest of the smaller 

 Pisidia, being a frequent and abundant inhabitant of ponds. 

 Among other localities we may specify Scarborough (Bean); 

 Preston (Gilbertson); Northumberland and Durham (Al- 

 der); Coggeshall, Essex (S. H.); Exmouth (Clark); near 

 Swansea (Jeffreys); Aberdeenshire (Macgillivray); near 

 Bantry Bay (Jeffreys) ; and, indeed, is universally distri- 

 buted in Ireland, where (as in Scotland likewise) Mr. 

 Thompson has met with it in marshy spots, adhering to the 

 same stones as the land mollusca, and occasionally has 

 taken it from moss only moistened by the spray of a water- 

 fall (Ann. N. H., vol. vi. p. 195). 



It inhabits Northern and Central Europe. 



