PISIDITJM. 9 



Hab. Specimens of this shell from which the above 

 description has been taken, are in the valuable collection 

 of Mr. Jeffreys, marked ^Exmouth, 1831, and Dr. Tur- 

 ton's Cabinet,' which seem to be the only specimens 

 ever found in Britain* 



PisiDiUM. Pfeiffer. 

 Shell — Suboval, equivalve, inequilateral, more or less 



concentrically striated. Hinge and lateral teeth 



similar to Cyclas. 

 Animal — 'Ma7itle produced behind into a single undimded 



tube. Foot tongue-shaped and very extensile. 



P. OBTUSALE. Pfeiffer. PL I, fig. 6. 



Shell globose, finely striated, with blunt projecting umbones. 



This is the smallest of British Pisidia, seldom exceed- 

 ing one ninth of an inch in length. The depth is equal 

 to the breadth, both being somewhat less than the 

 length. It may be distinguished by its swollen valves, 

 which give it a roundish oval shape. Umbones nearly 

 central. It is generally covered with a rough epider- 

 mis of a greenish black colour, margined by a yellowish 

 zone, which, when removed, presents a glossy pale yel- 

 low surface. 



Hab. It occurs not unfrequently in Cambridgeshire 

 and Oxfordshire, inhabiting small splash pools, and 

 other stagnant waters, and though not common in Scot- 

 land or Ireland, several localities in both countries have 

 been recorded. 



P. PUSiLLUM. Turton. PI. I, fig. T. 



Shell orbicular, oval, scarcely inequilateral, slightly striated; um- 

 bones broad and but slightly prominent 



Cyclas fontinalis, Drap, 8^c. Cyclas pulsilla, Ttirt. 



This species approaches nearest in form to P. obtusale, 

 the umbones being nearly central; but it is more com- 

 pressed and less glossy, Avhen the epidermis, common 

 to most of the species, has been removed. Its colour is 

 generally a tawny white. It sometimes attains a seventh 

 of an inch in length and an eighth in breadth. 

 * Forbes and Hanley's " British Mollusca," Vol. II, pp. 118, 119. 



