NATURAL SCIENCES OF PHILADELPHIA. 143 



As this shell appears to agree in so many characters with Corlula, the pro- 

 priety of separating it from that genus may be doubted by some; it presents, 

 however, such a marked difference from any species of Corbula we have seen, in 

 the peculiar very strongly projecting internal plate above referred to, that we 

 are satisfied there nmst have been some corresponding difference in the organi- 

 zation of the animal. 



As we have hitherto met with but the one species, it is of course impossible 

 to determine, in every instance, which of its characters are of specific, and which 

 of generic importance, consequently we will not attempt to give a formal 

 diagnosis of the genus, but will merely repeat, with slight modifications, the 

 description given by us some little time back in the Proceedings of the 

 Academy. 



CORBULAMELLA QREGAREA. 



Corbula ? gregarea^ M. ^ JT., Proceed. Acad. Nat. Set. Vol. viii, ^;. 84. 



Shell very small, inequivalve, approaching subglobose, a triangular form, 

 right valve more ventricose than the other ; beaks nearly central, gibbous, that 

 of the right valve elevated above the left, incurved ; surface polished and 

 marked with a few faint irregular concentric wrinkles of growth ; hinge having 

 under the beak a single prominent tooth in each valve ; anterior muscular 

 attachment rather indistinct, posterior raised upon a strongly projecting 

 lamina ; pallial impression scarcely sinuous. Length .13 inch ; height .13 inch ; 

 breadth .10 inch. 



Locality and position. Occurs in great numbers on Yellow Stone River in a 

 bed apparently a blending of No. 4 and 5. 



Cyprina arenarea. 



Cast of interior very broad ovate, subtrigonal, moderately compressed, most 

 convex in the umbonial region ; extremities rounded, the posterior sometimes 

 slightly truncate ; base semi-ovate or broadly rounded ; dorsal margin sloping 

 rather abruptly, with a regular convex curve from the beaks to the posterior 

 end, concave and much more abruptly' declining in front ; beaks rather elevated, 

 slightly gibbous, located a little in advance of the middle of the shell. Length 

 .95 inch ; height .80 inch ; breadth .53 inch. 



This species has much the form and general appearance of a Cytherea, but as 

 there is no indication of a sinus in the pallial impression, on any of the casts 

 we have seen, and the teeth of the hinge, as near as can be ascertained from 

 internal cast, are like those of Cyprina^ we place it provisionally in that genus. 



Locality and position. Big Sioux River, two miles above the mouth, in sand- 

 stone of formation No. 1, where it is associated with fragments of fossil wood 

 bored by Teredo. 



Cyprina cordata. 



Shell thick, cordate, gibbous ; extremities and base rounded ; umbonial region 

 ventricose ; beaks much elevated, nearly central, distinctl}' curved obliquelj'^ 

 inwards and forwards ; muscular impressions not very deep, surface marked by 

 strong concentric lines, and small wrinkles of growth, crossed by faint traces 

 of radiating lines. Length 1.66 inches ; height 1.68 inches ; breadth about 1.49 

 inches. 



As we have not yet seen the hinge of this shell, we are in some doubt in 

 regard to its affinities ; and only place it provisionally in the genus Cyprina. 

 The character of the muscular impressions, with its simple pallial line, and gen- 

 eral appearances, are such as to indicate its relations to that genus. It will be 

 at once distinguished from all the other forms we have yet seen from the Ne- 

 braska rocks, by its high curved beaks, gibbous cordate form, and distinct lines 

 of growth ; the radiating lines mentioned above are so ver}' faint as to be readily 

 overlooked, excepting when the shell is viewed in a certain light, and may be 

 often absent. 



Locality and position. Moreau River, No. 5 of the series. 

 1857.] 



