INVERTEBRATE PALEONTOLOGY". 101 



in crossing the hinge, so as to present their concave sides toward the ex- 

 tremities of the shell. 



The surface of slightly-worn specimens shows some faint indications of 

 fine, radiating striae ; but these perhaps belong more to the inner laminae, 

 which are very distinctly striate, than to any external surface-markings. Its 

 larger size, and other well-marked differences, will distinguish this shell from 

 all the Upper Missouri species of this genus yet known. 



It is very similar in form to N. obtusa of Fitton, as figured by d'Orbigny 

 (Pal. franchise, III, pi. 300, figs. 1-3); but, in addition to being rather 

 less compressed, it differs in having the middle portions of the lunule-like 

 area nearly flat, instead of carinate. Its internal cast would also be more 

 elevated at the beaks, while its anterior muscular impression is narrower and 

 located closer up under the hinge, as well as farther back. Its truncated 

 posterior dorsal margin is also decidedly more concave in outline. 



Locality and position. — Great Bend of the Missouri; in the lower part 

 of the Fort Pierre group. 



\ in ul:i | > I : i 1 1 i I II : a I • g i n a t i i . M . & H. 



Plate 15, figs. 8, a, b; and pi. 28, fig. 16. 

 Nucula plammarginata, Meek and Haydeu (Ap., 1856), Proceed. Acad. Nat. Sci. Philad., VIII, 85. 



Shell transversely subovate or subelliptic, compressed ; posterior or 

 shorter side obliquely truncated above, and abruptly rounded or subangular 

 below the middle ; anterior or longer side cuneate and rather narrowly 

 rounded ; base forming a regular semi-elliptic curve, not crenate within ; 

 dorsum declining gently with a gradual convex curve, from near the beaks to 

 the anterior extremity ; beaks small, incurved, nearly contiguous, and located 

 about half way between the middle and the posterior side ; surface marked 

 by very fine, irregular, radiating, and minute concentric striae ; hinge forming 

 at the beaks an angle of about 110°, having in the adult some twenty-six or 

 twenty-seven denticles on the longer or anterior side of the beaks, and about 

 ten behind, in each valve ; lunule-like area behind the beaks lance-ovate, 

 flattened along each side, and a little convex in the middle. 



Length, 0.95 inch ; height, 0.66 inch ; convexity, 0.38 inch. 



The substance of the shell in this species is of moderate thickness, and 

 generally less brilliantly pearly than that of the next. In most of our speci- 



