146 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



In the Tertiary rocks, we also meet with species of well-marked Cyprina, 

 and it is probable that, as here defined, this genus attained its greatest devel- 

 opment during the Tertiary epoch ; though even then the species were not 

 very numerous. At present, but the single, typical species is known to 

 inhabit our existing seas. It occurs in the Northern Atlantic, both on the 

 American and European coasts, being generally found near where rivers 

 empty into the ocean, 



Fig. 8. 



Cy priii a ovala, M. & H 



Plato 29, figs. 7 a, I>, c, and accompanying cut. 

 Cyprina ova'a, Meek and Haydeu (1857), Proceed. Acad. Nat. Sci. Philad., IX, 144. 



Shell transversely ovate, com- 

 pressed or moderately convex, rather 

 thin; extremities somewhat narrowly 

 rounded ; basal margin semi-oval ; 

 dorsal border declining with a gently 

 convex outline posteriorly, and more 

 abruptly in front; beaks not much 

 elevated, rather small, moderately 

 oblique, and located about half-way 

 between the middle and the anterior 

 Cyprina ovata. end; surface marked by distinct lines 



Hinge-view of right valve, to show details hetter 



igut valve, to snow details better r , , i 

 than the figure ou the plate. It is still faulty, how- 01 g rowrn - 

 ever, in making the posterior cardinal tooth appear 

 so deeply furrowed as to look rather like two teeth 



Length, 2.25 inches ; height, 1.95 

 xaoue . inches; convexity, 1.15 inches. 



The only good specimen of this species that I have seen consists of a 

 single right valve, which is entirely free from the matrix, and shows the 

 hinge and other internal characters quite as distinctly as we usually see them 

 in recent shells. Its hinge agrees almost exactly with that of the recenl 

 C. Islandka, from which the species differs mainly in being thinner, more 

 compressed, and more transversely oval in outline. Its posterior muscular 

 impression is broad-oval or subcircular, shallow, and apparently sinuous or 

 indented on the inner side ; while the anterior is narrower oval, and a little 

 more deeply impressed. The small anterior pedal scar is well defined just 

 above (he anterior adductor, with which it is connected exactly as in the 



