ioi8 



FRESH-WATER BIOLOGY 



165 (104) Interior of shell non-nacreous or porcellanous, or the whole shell 



of a prevailing prismatic substance 166 



Five families: 166, 167, 168, 173, i74- 



166 (167) Shell of a prevailing prismatic substance, mytiliform, very in- 



equilateral; beaks compressed, terminal; Ugament subinter- 

 nal; anterior adductor and pedal protractor muscles inserted 

 on a septum in the beak. Byssiferous. 



Family Dreissensiidae. 

 Only one genus Congeria Partsch. 



Represented in our fauna by two species. Example, C. 

 leucophaeata Con. (Fig. 1527; X 2), found on the Atlantic 

 coast from Maryland to Florida. 



Fig. 1527. 



167 (168) Shell porcellanous, subtrigonal, thick, and solid; ligament external; 

 hinge with true cardinal teeth and with both anterior and 

 posterior laterals; pallial line with a distinct sinus. 



Family Cyrenidae. 

 Only a single genus Cyrena Lamarck. 



Represented in our fresh-water fauna by a single species, C. 

 carolinensis Bosc (Fig. 1528), found in streams and brackish water 

 near the coast from South Carolina to Texas. 



Fig. 1528. 



168 (173) Shell non-nacreous, usually small and thin; hinge with cardinal and 



both anterior and posterior lateral teeth; no hinge plate; 

 pallial Hne simple. . . . Family Sphaeriidae . . 169 

 Four genera: 169, 170, 171, 172. 



169 (170) Shell oval, equilateral; beaks nearly subcentral; nepeonic valves 



not distinctly separated from the subsequent growth of the 

 shell; cardinal teeth two in each valve. . Sphaerium ^co^oM. 



This group contains the largest species of the 

 family and is easily distinguished from Musculium by 

 the thicker, striate shell and noncalyculate beaks. 

 The species are numerous and of general distribution. 

 Example, S. simile Say (Fig. 1529; X if). 



Fig. 1529. 



