SPHJERIUM. 



47 



distinct, in the shape of the letter V reversed ; lateral teeth elongated ; 

 valves delicate, interior light blue ; strife hardly 

 visible ; epidermis glossy, light greenish horn color, Fig. 43. 



with at times a zone of bright yellow on the in 

 ferior margin. 



Long. 0.50; Lat. 0.43; Diam. 0.18 inches. 

 Hab. North America, in the region of Lake Su 

 perior ? Iowa and Canada. (Cabinets of Agassiz, 

 Smithsonian Institution, Jay, Garden of Plants in 

 Paris, and Prime.) s , j a yanum. 



Cyclas jayensis, PRIME, Bost. Proc. IV, 1851, 157. 



This attractive and rare species is easily distinguished by its 

 elevated shape and by its abrupt lateral margins, which give it a 

 somewhat triangular appearance. It is related to S. ryckholti of 

 Europe, from which it differs, however, in being more inflated, its 

 beaks are less prominent, the shell is more elevated, and its 

 anterior margin is abrupt, whereas in S. ryckholti it is distended 

 and angular. 



Fig. 44. 



S. tenue. 



21. Spltaermm tenue, PRIME. Animal not observed. 



Shell small, transversely oblong, pellucid, moderately full, subequi- 

 lateral ; 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. 0.18; Lat. 0.12; Diam. 0.06 inches. 



Hab. North America, in the Androscoggin, Maine, and in the Upper 

 Mackenzie, British America. (Cabinets of Prime and the Smithsonian 

 Institution.) 



Cyclas tenuis, PRIME, Bost. Proc. IV, 1851, 161. 



This species, the smallest one known to inhabit the United 

 States, was discovered some years since by Mr. Girard, from whom 

 I obtained my specimens. 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 character 

 istics of a mature shell. In outline it seems to be allied to S. 

 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. 



