30 PROCEEDINGS OP THE ACADEMY OF 



by a regular curve into the anterior margin, but curving suddenly behind so 

 as to form an obtuse angle, causing the posterior side to appear broader, thus 

 giving the shell a somewhat rhombiform appearance ; cardinal teeth strong, 

 assuming the shape of the letter V reversed ; lateral teeth very much 

 elongated. 



The young shell is more compressed than the adult ; it is usually light 

 yellow. 



Long. 8-16; lat. 7-16; diam. 516 inches. 



Hah. N. America, in the States of Maine, Vermont, Massachusetts, New 

 York, New Jersey, Pennsylvania, Ohio, S. Carolina, Georgia, Mississippi, 

 Alabama and Arkansas. (Cabinets Acad. Nat. Sci. Phila., Boston Soc. Nat. 

 Hist., Garden of Plants at Paris, Agassiz, Jay and Prime.) 



This species varies much according to the localities where it is found, which 

 accounts in part for the number of names it has received. The Cyclas 

 orbicularia, of which I have authentic specimens from Mr. Barrat, is a 

 genuine Sph. partumium, without even any local modifications of shape. 

 The Cyclas mirabilis, from Georgia, is a small form of this species, and 

 the Cyclas c oe r ul e a differs from the type in being a little less inflated. The 

 Cyclas e b u r n e a , from Arkansas, varies from the Northern Sph. partn- 

 mium in being more compressed and a little more elevated. I do not think, 

 however, taking the difference of localities into consideration, that these are 

 characters sufficient to warrant retaining the Cyclas eburnea as a distinct 

 species. I had an opportunity while in Paris to assure myself that the variety 

 No. 2 of Cyclas cornea was a true Sph. partumium. 



This species is not only very widely distributed, but where it is found, it 

 occurs in large numbers. The only one of our Northern species to which it 

 bears much resemblance is the Sph. truncatum, and that is only in general 

 outline ; the Sph. partumium is much more inflated and transversely 

 more broad. 



18. Sph. J a y a n u m , Prime. 



Cyclas Jayensis, Prime, Bost. Proc. iv. 157, 1851. 



Animal not observed. Shell rhombic, nearly equilateral, moderately con- 

 vex, thin, fragile, somewhat translucent, drawn up to an angle towards the 

 hinge margin ; anterior and posterior margins very abrupt, inferior very 

 slightly curved ; beaks central, calyculate, approximate at apex ; hinge margin 

 considerably shorter than the basal margin, slightly curved ; cardinal teeth 

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

 delicate, interior light blue ; striae hardly visible ; epidermis glossy, light 

 greenish horn color, with at times a zone of bright yellow on the inferior 

 margin. 



Long. 8-16 ; lat. 7-16 ; diam. 3-16 inches. 



Bab. N. America, in the region of Lake Superior ? (Cabinets Agassiz, Jay, 

 Garden of Plants in Paris, and Prime.) 



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 the Sph. 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 Sph. 

 Ryckholti it is distended and angular. 



19. Sph. subtransversum, Prime, Proc. Zool. xxviii. 322, 1860. 



Animal not observed. Shell small, transversely oblong, equilateral, trans- 

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

 cate ; epidermis greenish yellow. 



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



Rob. 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. 



[Feb. 



