CYPEINID^. 181 



diverging ; in Sphserium, tubercular or of columnar shape, placed 

 obliquely toward each other. 



EUPERA, Bourg., and fluminea, rivulina and fossarina, 

 Clessin, are names of sections of the genus ; they have but little 

 distinctive value. 



Cyrenoides, Joannis, 1835. 



Syn Cyrenella, Desh., 1838. 



Distr. — 4 sp. River Senegal. The marine species are Diplo- 

 dontse. Fossil, 1 sp. Europe. C. Diqjontn, Joannis (cxiv, 49). 



Shell orbicular, ventricose, thin, eroded at the beaks ; epidermis 

 dark olive ; ligament external, prominent, elongated ; cardinal 

 teeth 3-2, the central tooth of the right valve bifid ; muscular 

 impressions long, narrow ; pallial line simple. 



Animal with mantle open in front and below, margin simple, 

 siphons short, united ; palpi moderate, narrow ; gills very un- 

 equal, narrow, united behind ; foot cylindrical, elongated. 



? Cycloconcha, Miller, 18T4. 

 Distr. — C. mediocardinaliSj Miller. L. Silur. ; U. S. 

 Shell nearly circular, equivalve, concentrically sculptured, with 

 a hinge-tooth and posterior and anterior laterals. 

 Its pertinence to this family is doubtful. 



(^Cardiacea.) 



Family CYPRINID^. 



Shell regular, equivalve, oval or elongated ; valves close, solid; 

 epidermis thick and dark ; ligament external, conspicuous ; 

 cardinal teeth three in each valve, and a posterior lateral tooth ; 

 pedal scars close to, or confluent with, the adductors ; pallial line 

 slightly sinuous ; siphons very short, with ciliated orifices ; foot 

 thick, linguiform. 



Cyprina, Lamarck, 1812. 



Etyra. — Kuprinos (from iTupWs) , related to Yenus. 



Syn. — Arctica, Schum., 181*7. 



Distr. — G. Islandica (cxiv, 50-52) ranges from Greenland and 

 the United States to the Icy Sea, Norway and England ; in 5-80 

 fathoms water. It occurs fossil in Sicily and Piedmont, but not 

 alive in the Mediterranean. Fossil, 90 sp. (D'Orbigny.) Mus- 

 chelkalk — ; Europe, U. S. 



Shell oval, large and strong, with usually an oblique line or 

 angle on the posterior side of each valve ; epidermis thick and 

 dark; ligament prominent, umbones oblique; no lunule ; cardinal 

 teeth 2*2, laterals — 1, 1 — ; muscular impressions oval, pol- 

 ished ; pallial sinus obsolete. 



Animal with the mantle open in front and below, margins 



