528 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



little above the. cardinal edge, located nearly half-way between the middle 

 and the anterior end ; surface marked by rather obscure, irregular lines of 

 growth ; lateral teeth well developed, the posterior being longer than the 

 anterior. 



Length, about 0.55 inch ; height, 0.36 inch ; breadth, 0.24 inch. 



The most striking peculiarities of this shell, are its long straight 

 hinge and small compressed and depressed beaks. These characters alone 

 will prevent it from being confounded with any of the species yet obtained 

 from these formations. In the straightness of the hinge-line, it resembles 

 the young of the recent S. sulcatum, Lam. (sp.), from which it differs in 

 having its beaks located farther forward, and smaller, as well as more com- 

 pressed. From the adult S. sulcatum, of its own size, it differs materially in 

 the ste-aightness of its hinge, less nearly central beaks, and more compressed 

 form. 



Locality and position, — Near Grand Island, on the Upper Missouri; 

 from the Judith River group of the Fresh- and Brackish-water Lignites: 

 probably belonging to the closing part of the Cretaceous period. 



CORBULID^E. 



Genus CORBULA, Bruguiere (see page 240). 

 Subgenus PACHYODON, Gabb (see page 241.) 



The following species, although not agreeing in all respects with the 

 typical forms of the Pachyodon group, seem to correspond in the forward 

 obliquity of their beaks, and some other characters, more nearly to that than 

 to any of the other sections of the genus yet proposed. 



Corbula maetriformis, M. & H. 



Plate 43, figs. 7, a-f. 



Corbula maetriformis, Meek and Haydeu (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 117. 

 Corbula (Potamomya) maetriformis, Meek aud Haydeu (18(i0), ib., XII, 432. — Meek (1866), in Conrad's 

 Smithsonian Check-List N. Am. Eocene Invert. Fossils, 8. 



Shell ovate-subtrigonal, moderately convex ; right valve a little shorter, 

 thicker, and more gibbous than the other; anterior side of both valves 

 obliquely subtruncated, slightly concave in outline above, and narrowly 

 rounded into the base below ; posterior side longer, and narrowing toward 

 the extremity, which is narrowly rounded, or very obscurely subtruncated; 

 base semi-ovate, being most prominent a little in front of the middle; beaks 



