50 AMERICAN CORBICULAD^J. 



but still intermediate forms uniting the two are so frequent that 

 it is not possible that they should form distinct species. 



C. crocea, Lewis, is a young of this species. 



Compared with S. sphsericum, the S. securis is more equi 

 lateral, the beaks are less tumid and less inclined, the sides are 

 less rounded, and the hinge-margin is less curved. 



25. Sphaerium rosaceum, PRIME. Animal not observed. 

 Shell small, rounded-oval, fragile, translucent, subequilateral, somewhat 



compressed, margins generally rounded ; beaks nearly cen- 

 Fig. 48. tral, slightly inclined towards the anterior, calyculate, 

 approximate at apex ; valves very slight, a little convex 

 in the region of the umbones ; striae regular, hardly visi- 

 k| e . e pi(j erm i s shiny, reddish-brown; hinge-margin nearly 

 S rosaceum straight, delicate, narrow ; cardinal teeth nearly obsolete, 

 lateral teeth slight, elongated. 



Long. 0.25 ; Lat. 0.18 ; Diam. 0.15 inches. 



Hob. North America, in the Schuylkill River. (Cabinet of Prime.) 

 Cyclas rosacea, PRIME, Bost. Proc. IV, 1851, 155. 



This species, which is very rare, the only specimens known to 

 me being those in my collection, is not very liable to be con 

 founded with others. Compared with S. occidental, it is less 

 full, the beaks are more prominent and are calyculate. 



26. Spliaerilim spliaericiim, ANTHONY. Animal not observed. 



Shell globose, subequilateral, transversely 

 Fig. 49. ova | . anterior side narrow, distended, rounded ; 



inferior margin rounded ; posterior margin sub- 

 abrupt ; beaks inclined towards the anterior, 

 large, prominent, calyculate ; valves slight, very 

 convex, interior blue; striae fine and regular; 



epidermis greenish ; hinge-margin much curved ; 

 S. sphcericum. cardinal teeth strong, united at base and dis 



posed in the shape of the letter V reversed ; 

 lateral teeth prominent, very distinct, rather short. 



Long. 0.31 ; Lat. 0.20 ; Diam. 0.18 inches. 



Hob. North America, in the Black River, Ohio. (Cabinets of Anthony, 

 Prime, and Smithsonian Institution.) 



Cyclas sphserica, ANTHONY, Bost. Proc. IV, 1852, 275. 



Yery rare ; I have seen but few specimens of this species besides 

 those in Mr. Anthony s collection and in mine. Compared with 





