Synopsis Frf;sh-Wati;r Moi^lusca 



85 



Family CYREMD^. 



Shell porcellanous, thick, solid, hinge-plate developed, teeth strong, 

 prominent; equivalve; ligament external. Siphons distinctly developed; 

 dioecious. 



Genus CYRENA Lamarck, 1818. 



Shell oval or subtrigonal, thick, solid, cardi- 

 nal teeth, three in each valve; two anterior and 

 two posterior laterals in the right valve and one 

 anterior, and one posterior in the left ; pallial line 

 (in the American species, Section Polymesoda 

 Rafinesque, 1820) with a deep, narrow sinus. 



Type : C. hengalensis Lam. 



Type : Section Polymesoda, Cyclas caroliniana 

 Bosc, fig. 225. 



Fis^. 22=; 



Family SPH^RIID^. 



Shell small and thin, ligament feeble, short ; pallial line simple ; no hinge 

 plate; cardinal teeth usually two in each valve; laterals four in the right 

 valve, two anterior and two posterior and two in the left, one anterior and 

 one posterior; foot long, narrow, grooved, byssiferous when young; mona'- 

 cious ; the young incubated in a marsupium formed by the inner gill. 



Key to the genera of Sphceriidce. 



r Shell nearly equilateral ; beaks subcentral 2. 



[ Shell inequilateral ; beaks terminal Pisidinm. 



Nepeonic valves not distinctly separated from the subse- 

 quent growth of the shell 3- 



2. < 



Nepeonic valves inflated, separated from the adult growth 



by a distinct sulcus Musculium. 



Shell oval, cardinal teeth, two in each valve Spharium. 



Shell rhomboidal, one cardinal tooth in each valve Eupera. 



Genus SPH^RIUM Scopoli, 1777. 



Shell thin, oval, more or less inflated ; subequilateral, beaks subcentral ; 

 surface smooth or concentrically striate, teeth small, cardinals two in each 

 valve, lateral teeth double in the right, single in the left valve. Siphons 

 united at the base, but double at the extremity. 



