INVERTEBRATE TAL^EONTOLOGY. 245 



extremity, which is subangular in outline; beaks small, nearly equal, pointed, 

 incurved, and located slightly in advance of the middle. Surface smooth, or 

 only obscurely undulated near the umbo of the right valve, and showing, in 

 some conditions under a good lens, very faint traces of obsolete radiating 

 striae near the thickened border. 



Length, 0.15 inch ; height, 0.1,") inch; convexity, 0.09 inch. 



This little shell is remarkable for having the pallial border, particularly 

 of the right valve, thickened so as to form a distinct marginal band; and for 

 being very nearly equivalve. The cardinal margin of the right valve seems to 

 overlap that of the other slightly ; but, in all other respects, their borders are 

 almost exactly equally prominent. It will be readily distinguished by its 

 obvious characters from all the other forms known in these formations; and 

 I know of no nearly-allied foreign species. It must be very rare, as only a 

 single specimen has yet been found. 



Locality and position. — Moreau River, Dakota Territory ; from the Fox 

 Hills group of the Upper Missouri Cretaceous scries. 



Corbnla inornata, M. & H. 



Plate 30, figs. 4, a, b, c, d. 

 Corbnla inornata, Meek and Haydeu (1856); Proceed. Acad. Nat. Sci. Philad., VIII, 52. 



Shell small, oval-subtrigonal, very gibbous, distinctly inequivalve; ante- 

 rior side of right or larger valve irregularly rounded or obliquely truncated 

 above, and subangular below ; anterior side of smaller valve more narrowly 

 rounded ; posterior side of both obliquely truncated above, and more or less 

 distinctly angular below ; basal margins of each nearly straight, or but slightly 

 convex in outline, a little warped laterally, and not crenulated within ; beaks 

 subcentral, that of right valve considerably elevated above the other, flattened 

 or depressed on top, incurved, and sometimes truncated at the point by press- 

 ure against that of the other valve. Surface nearly smooth, or only marked 

 by very obscure lines of growth, and a few indistinct, irregular undulations. 



Length, 0.27 inch; height of right valve, 0.23 inch; height of left valve, 

 0.20 inch ; convexity of the two valves, 0.22 inch. 



The posterior umbonal slopes are obtusely angular from the beaks quite 

 down to the postero-basal border, and within this angle »n the truncated 

 posterior portion of each valve, but more distinctly on the right, a narrow 



