NATURAL SCIENCES OP PHILADELPHIA. 179 



below ; antero-ventral side provided with a large hiatus, formed by a similar, 

 but deeper rectangular notch, which extends from the base nearly half way up 

 to the beaks, and back almost to the middle of the valves ; base, between the 

 anterior and posterior notches, extended downward in the form of a narrow 

 prolongation, which curves under, and is the only part of the ventral borders 

 of the two valves that come in contact ; beaks elevated, gibbous, incurved, 

 and located between the middle and the anterior margin ; surface ornamented 

 by small concentric lines, which are curved, and deflected parallel to the great 

 irregularities of the free borders, and crossed by two distinct radiating grooves, 

 one of which passes from the back part of the beaks^obliquely downward and 

 backward to the corner of the posterior notch, ari^the other nearly directly 

 downward to the extremity of the ventral prolongation. Length, of a medium 

 sized specimen, 0-16 inch ; height 0-14 inch ; gibbosity 0-13 inch. 



Locality and position. Fort Clark, on the Missouri, in formation No. 5. 



Mactra Siouxensis, M. & H. Internal cast oval-subtrigonal, moderately 

 gibbous ; anterior border narrowly rounded ; posterior margin subangular at 

 the extremity ; base forming a nearly semiovate curve, the most prominent 

 part of which is in front of the middle ; dorsal outline declining with a slightly 

 convex outline behind the beaks, and distinctly concave in front of them ; beaks 

 prominent, rather gibbous, very nearly central ; pallial impression provided 

 with an oval sinus, which appears to be a little narrower behind than in tHe 

 middle, rounded at the anterior extremity, and extending nearly in a horizontal 

 direction, about three-fourths of the way towards the middle of the valves. 

 Length 1-55 inches ; height, 1*22 inches ; convexity 0-76 inch. 



Locality and position. Near mouth of Big Sioux River, in formation No. 1, 

 of the Nebraska Cretaceous series. 



Mactra gracilis, M. & H. Shell small, rather thin, ovate-subtrigonal, 

 moderately gibbous, anterior end rounded, a little broader than the other ; 

 base forming a broad semiovate curve, being usually more prominent towards 

 the front than behind ; posterior margin rather narrowly rounded, or sub- 

 truncate ; beaks moderately prominent, and located slightly in advance of the 

 middle ; escutcheon comparatively large, lance-ovate in form ; surface marked 

 by distinct, regular lines of growth : hinge unknown. Length 0-49 inch ; 

 height 0-38 inch; convexity about 0-24 inch. 



Locality and position. On Yellowstone River, 150 miles above the mouth, in 

 beds containing a mingling of the fossils of No. 4 and 5. 



Tellina? Formosa, M. & H. Shell subelliptical, very thin, moderately con 

 vex ; anterior extremity a little wider than the other, but very narrowly round- 

 ed ; posterior side subangular at the extremity ; base forming a semi-elliptical 

 curve ; dorsum sloping gradually, with a slightly convex outline in front and 

 rear; beaks small, and located almost exactly in the middle; surface marked 

 by rather obscure, irregular lines of growth, and extremely fine radiating striae, 

 only visible by the aid of a magnifier ; hinge unknown. Length 0*67 inch ; 

 height 0-40 inch; convexity (of a right valve) about 0*13 inch. 



Locality and position. Twenty miles below mouth of Cannon Ball River, 

 formation No. 5. 



Cyprina humilis. M. & H. Shell ovate, gibbous, thick, very oblique ; ante- 

 rior margin scarcely extending beyond the beaks, abruptly rounded below ; 

 base semiovate in outline, most prominent towards the front, sometimes a little 

 contracted behind ; posterior extremity rounding obliquely, with a broad curve 

 from the dorsum to the postero-basal extremity, which is narrowly rounded ; 

 beaks very oblique, almost overhanging the anterior border, declining and 

 turned a little inwards at the extremities ; umbonal slopes prominent from near 

 the beaks obliquely backward to the lower part of the anal margin ; surface 

 marked by distinct, subimbricating lines of growth. Length 1-70 inches ; 

 height 1-34 inches; breadth 1-30 inches. 



I860.] 



