134 CYCLADID^. 



vaceous ash-colour, and is broadly zoned at the margin of 

 the larger examples with a paler or yellowish tint. It 

 is sometimes, though rarely, observes Mr. Jenyns, of 

 an uniform brownish or yellowish hue ; sometimes, 

 too, ashy-brown with lighter spots. The ventral margin 

 is moderately curved behind, but arcuated and much 

 ascending in front. The anterior side, which is much 

 produced, is compressed and tapering at its extremity, 

 where it is almost symmetrically rounded ; the upper edge, 

 however, is rather more sloping and slightly less convex. 

 The hinder side is extremely short, and its termination is 

 very bhmt, the posterior edge being but little convex, and 

 almost forming a rounded off rectangle with the basal mar- 

 gin ; the upper corner also, from the straightness of the 

 dorsal edge, which has a slight but distinct declination, is 

 subangulated. The ligament is but little conspicuous ; the 

 umbones are not broad, nor much prominent. The interior 

 is white with a bluish cast. 



The dimensions of the full-grown shell are ordinarily 

 five lines and a quarter in length, and three lines and three- 

 quarters in breadth. 



The animal is white, the siphon rather variable in shape, 

 sometimes abbreviated, sometimes subconic ; the apex is 

 obliquely truncated, and often more or less recurved. The 

 foot is often protruded to a great length as compared with 

 the size of the shell. 



Although not so frequent as Cyclas cornea^ this is one of 

 our most abundant fluviatile shells. Dr. Maton, who first 

 defined it as a distinct member of our Fauna, obtained his 

 specimens from the Avon, near Salisbury, and Montagu 

 from the noi-th Avon : it is extremely plentiful in the New 

 River, near London (S. H.), where it partially imbeds it- 

 self in the mud, never rising (as certain other Pisidia) to 



