118 CONCHACEA. 



Shell thin, rhombic-orbicular, subequilateral, somewhat com- 

 pressed ; sides rather obtuse, rendering the general contour 

 more circular than others of the genus ; obsoletely striate ; 

 beaks obtuse, of a grayish ash-colour. 



It is much more compressed than any of the other British 

 Cyclyda?. 



Found in Ulliswater; a small pond on the grounds at Corby, 

 Cumberland ; and at Prestwick Car, Northumberland. It is a 

 rare British shell. Thompson says it is rare in Ireland, but has 

 been met with at Tallagt, by Mr. Ball, and in the pond, Phoenix 

 Park, by Mr. Warren. 



4. Cyclas calyculata, pi. XXIII, f. 6. 



Cyclas calyculata, Drapernaud, Hist, des Moll., p. 130, pi. 

 10, f. 14, 15; Lamarck, V, p. 559, No. 5; Pfeiffer, p. 122, pi. 5, 

 f. 17, 18; Turton, Man., p. 14, f. 3; Forbes, p. 50; Brown, 

 lUust. Conch., p. 94, pi. 39, f. 21 ; lb., First Ed., pi. 17, f- 13 ; 

 Cyclas lacustris, Alder, Mag. Zool. and Bot., II, p. 118. 



Shell thin, rhombic-orbicular, grayish ash-coloured, compres- 

 sed, diaphanous, substriate, or nearly smooth ; slightly angular 

 on one side ; umbones very prominent, and acute. About a 

 quarter of an inch long; and its width nearly the same. 



Found in the lakes of Westmoreland, and is not uncommon 

 in several pits and ponds near Manchester, but is a very rare 

 and local species. 



Genus V.—PISIDIUM.— Pfeiffer. 



Shell equi valve, transverse; sides unequal, completely closing; 

 in the right valve one, and in the left valve two opposite, very 

 small, primary teeth ; behind and before, two thin, lamellar, side 

 teeth; those of the latter cleft in the right valve, in order to 

 receive the opposite ones. 



The shells of this genus are distinguished from those of 

 Cyclas, by being less equilateral in their form, and in the liga- 

 mentary side being shortest, which is the reverse in Cyclas. 



1. PiSIDIUM OBLIQUUM, pi. XXIII, f. 7. 



Pisidium obliquum, Pfeiffer, p. 124, pi. 5, f. 19, 20; Brown, 

 Illust. Conch., p. 94, pi. 39, f. 22 ; lb., First Ed., pi. 17, f. 14 ; 

 Cyclas obliqua, Lamarck, V, p. 559, No. 4 ; Cyclas paliistris, 

 Drapernaud, Hist, des Moll., p. 131, pi. 10, f. 17, IS; Cyclas 



