92 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



Trigonarca (Brtviarca!) S i o u x e n s i s, H. & M. 



Plate 1, fig. G. 



Pcctunculua Siouxensis, Hall aud Meek (1854), Memoirs Am. Acatl. Arts and Sci., V, 384. — Gabb (1861), 

 Synop. Moll. Cret. Form., 103. 



Jjiiiuii Shiiueiisis, Meek (lst>4), Smithsonian Check-List Cret. Invert. Foss. N. Am., 8. 



Shell (as inferred from internal casts) subquadrangular, gibbous, a little 

 longer than high; basal margin rather straight; anterior outline vertically 

 subtruncated, but convex along the middle, and rounding rather abruptly into 

 the base, as well as to the hinge above; posterior side obliquely truncated 

 abovej and narrowly rounded below ; hinge-margin a little straightened along 

 the middle, but curving downward at each end ; hinge-denticles small and 

 rather crowded ; beaks nearly central, and slightly or not at all oblique. 

 Cardinal area and surface-markings unknown, 



Length, about 0.93 inch; height, 0.84 inch. 



We yet only know this species from internal casts, which, of course, give 

 no idea of the nature of its surface-markings or of its hinge-area. There can 

 scarcely be any doubt, however, from its obliquely-truncated posterior margin, 

 rather gibbous form, and general aspect, that it belongs more properly to the 

 genus Trigonarca than to Axincea; while the fineness of its hinge-denticles 

 rather indicates for it a place in the Breviarca section, than with the typical 

 trigonarcs. 



As we have not succeeded in finding any specimens as well preserved as 

 that originally figured in the paper cited above, it has been thought preferable 

 to give merely a copy of the original figure on our plate. 



Locality and position. — Mouth of Big Sioux River, on the Missouri; 

 from the Dakota group of the Upper Missouri Cretaceous series. 



Trigonarca (Breviarca?) Salinaensis, Meek. 

 Plate 2, figs. 1, (i, b, c. 



Shell small, suborbicular, gibbous ; rounded- subtruncate anteriorly, semi- 

 ovate below, and apparently slightly truncal ed behind; hinge-line declining 

 rather distinctly from the beaks; hinge-denticles comparatively rather strong, 

 about eight of them being seen on what appears to be the anterior side of the 

 beaks in one valve; muscular impressions very faintly marked; beaks promi- 



