13 



lucent, fragile, compressed ; beaks central, large, calyculate ; strise very deli- 

 cate ; epidermis greenish yellow. 

 Long. 3-10 ; lat. 2-10 ; diam. 1-10 inches. 

 Hob. N. America, at Tabasco in Mexico. (Cabinet Cuming.) 

 The only specimen I have seen of this species was sent to me for description 

 by Mr. Cuming. 



20. Sph. argentinum, D'Orbigny. 



Cyclas argentina, D'Orb., Mag. de Zool. 1835.. Voy. en Amer. Merid. 

 5CJ8, pi. 83, f. 57, 1844. 



Animal not observed. Shell oval, small, translucent, compressed ; anterior 

 side short, somewhat angular, posterior side distended and truncated at the 

 end ; beaks calyculate ; striae delicate ; epidermis greenish brown ; valves 

 slight, interior bluish ; cardinal teeth united, lateral teeth hardly visible. 



Long. 5-16 ; lat. 4-16 inches. 



Hab. S. America, at Montevideo at the base of the Cerro. (Cabinet British 

 Museum.) 



It has not been my good fortune to meet with this species. M. D'Orbigny 

 says it bears some resemblance to the Cyclas caliculata, meaning thereby, 

 I presume, the shell now known to European conchologists under the name of 

 Sph. lacustre, Ferussac. 



21. Sph. tentte, Prime. 



Cyclas tenuis, Prime, Bost. Proc. iv. 161, 1851. 



Animal not observed. Shell small, transversely oblong, pellucid, mode- 

 rately full, subequilateral ; anterior and basal margins rounded, posterior 

 margin subabrupt; beaks nearly central, not prominent, calyculate ; striations 

 very fine and regular, hardly perceptible ; epidermis glossy, light straw color ; 

 valves slight, interior straw color ; hinge margin short, narrow, nearly straight ; 

 cardinal teeth very diminutive, lateral teeth small, elongated. 



Long. 3-16; lat. 2-16; diam. 1-16 inches. 



Hab. N. America, in the Androscoggin, Maine. (Cabinet Prime. ) 



This species, the smallest one known to inhabit the United States, was dis- 

 covered some years since by Mr. CKrard, from whom I obtained my specimens, 

 the only ones I have met with. It may possibly be the young of some species, 

 but if so, it would be very difficult to say which ; setting aside its diminutive 

 size, it appears to have all the characteristics of a mature shell. In outline it 

 seems to be allied to the Sph. transversum; it is, however, more inflated, 

 less elongated, and its margins are more rounded. At first sight, it might 

 readily be mistaken for a Pisidium. 



22. Sph. transversum, Say. 



Cyclas transversa, Say, New Harm. Dissem. 2, 356, 1829. 

 C. detruncata, Prime, Bost. Proc. iv. 155, 1851. 

 C. gracile, " loc. sub. cit. iv. 156, 1851. 



C. constricta, Anthony, " " " iv. 274, 1852. 



Animal white, syphonal tubes pink, foot white. Shell transversely oblong, 

 elongated, subinequilateral, translucent; anterior side narrow; anterior 

 margin rounded, posterior margin subtruncate, basal very much curved ; 

 beaks placed somewhat on the anterior side, large, calyculate, very much 

 raised above the outline of the shell ; striae very delicate ; epidermis greenish 

 yellow, of a darker shade at times on the region of beaks ; valves slight, 

 interior bluish ; hinge margin very nearly straight, narrow ; cardinal teeth 

 compressed, in the shape of the letter V reversed, and very much expanded ; 

 lateral teeth slightly elongated. 



Long. 10-16 ; lat. 7-16 ; diam. 4-16 inches. 



Hab. N. America, in the States of New York, Pennsylvania, Ohio, Ken- 

 tucky and Arkansas. (Cabinets Jay and Prime.) 



This large and delicate species is remarkable for its very transverse shape 

 and for the narrowness of the anterior extremity as compared to the posterior. 



