IT 



Found plentifully at Cambridge, Mass. I cannot see differences sufficient be- 

 tween the Sph. s e c u r i s and the Cyclas cardissa to separate them ; the 

 Cyclas cardissa is more globose, transversely shorter, more elevated, but 

 still intermediate forms uniting the two are so frequent that it is not possible 

 that they should form distinct species. 



The Cyclas c r o c e a , Lewis, is a young of this species. 



Compared to the Sph. sphsericum, the Sph. securis is more equilateral, 

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

 hinge margin is less curved. 



31. Sph. rosaceum, Prime. 



Cyclas rosacea, Prime, Bost. Proc. iv. 155, 1851. 



Animal not observed. Shell small, rounded-oval, fragile, translucent, sub- 

 equilateral, somewhat compressed, margins- generally rounded ; beaks nearly 

 central, slightly inclined towards the anterior, calyculate, approximate at 

 apex ; valves very slight, a little convex in the region of the umbones ; strise 

 regular, hardly visible ; epidermis shiny, reddish-brown ; hinge margin nearly 

 straight, delicate, narrow ; cardinal teeth nearly obsolete, lateral teeth slight, 

 elongated. 



Long. 4-16 ; lat. 3-16 ; diam. 5-32 inches. 



Hab. N. America, in the Schuylkill River. (Cabinet Prime.) 



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

 those in my collection, is not very liable to be confounded with others. Com- 

 pared to the Sph. o c c i d e n t a 1 e, it is less full, the beaks are more prominent 

 and are calyculate. 



32. Sph. sphaericum, Anthony. 



Cyclas sphcerica, Anthony, Bost. Proc. iv. 275, 1852. 



Animal not observed. Shell globose, subequi- 

 lateral, transversely oval ; anterior side narrow, 

 distended, rounded ; inferior margin rounded ; pos- 

 terior margin subabrupt ; beaks inclined towards 

 the anterior, large, prominent, calyculate ; valves 

 slight, very convex, interior blue ; strise fine and 

 regular ; epidermis greenish ; hinge margin much 

 curved ; cardinal teeth strong, united at base and 

 disposed in the shape of the letter V reversed ; 

 lateral teeth prominent, very distinct, rather short. 

 Long. 5-16 ; lat. 9-32 ; diam. 3-16 inches. 



Hab. N. America, in the Black River, Ohio. (Cabinets Anthony and 

 Prime.) 



Very rare ; I have never seen any specimens of this species but those in 

 Mr. Anthony's collection and in mine. Compared to the Sph. rosaceum. 

 it is less equilateral, more inflated and the margins are less rounded. 



33. Sph. truncatum, Linsley. 



Cyclas calyculata, C. B. Adams, Amer. II., xi. 277, 1841. 

 " truncata, Linsley. Amer. II., N. Ser., vi. 234, f. 3, 1848. 

 " pellucida, Prime, Boston Proc., iv. 277, 1852. 



Animal not observed. Shell rhombic-orbicular, lenticular, thin, pellucid, 

 very slightly inflated, subequilateral : anterior side narrower ; anterior mar- 

 gin rounded ; posterior margin nearly a straight line ; basal somewhat curved ; 

 beaks central, calyculate, approximate at apex ; strise very delicate ; epider- 

 mis glossy, light greenish horn color ; valves slight, very little convex ; inte- 

 rior light blue ; hinge margin very nearly straight ; very narrow ; cardinal 

 teeth diminutive, united at base ; lateral teeth slight, narrow, not much 

 elongated. 



