34 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



* 



elevated above the hinge-margin ; hinge-marngin straight, less than the length 

 of the .shell, and pointed at the posterior extremity; anterior extremity short, 

 rounded; posterior margin obliquely truncate; no line of demarkation 

 between the wing and the body of the shell ; basal margin forming an 

 elliptic curve; surface marked by sharp, strong ribs, with sometimes an 

 intermediate smaller one, crossed by faint concentric undulations, and 

 parallel fine lines of growth. 



"Length, 0.30 inch; height, 0.22 inch; hinge-line making an angle 

 with the posterior slope of about 129°.'' 



Having no good specimens of this species for study, I have merely 

 copied the original figure and description. Until the right valve can be seen, 

 it will not be possible to determine whether or not this shell belongs to the 

 group Oxytoma, but I think it most probably does not. 



Locality and position. — The specimen first figured by Professor Hall 

 and the writer, was collected at the Great Bend of the Missouri, below 

 Fort Pierre; in the lower part of the Fort Pierre group, formation No. 4 of 

 the Cretaceous. The only specimens of it I have since seen, came from the 

 same place. 



Ptcria (Oxytoma) Nebrascana, E. & S. 



Plate 16, figs. 3, a, b ; and pi. 28, fig. 11. 



Avicula Nebrascana, Evans and Shnmard (1857). Trans. St. Louis Acad. Sci., I, 38. 

 Pteria Nebrascana, Meek (1864), Smithsonian Check-List Cret. Fossils N. Am., 9. 



Shell small, rather compressed, obliquely oval exclusive of the wings, 

 distinctly inequivalve, the left valve being more convex, with its ventral mar- 

 gin and posterior wing projecting apparently beyond those of the other ; 

 hinge generally, if not always, less than the length of the valves. Left valve 

 with anterior wing small, triangular, compressed, generally about rectangular 

 or sometimes a little more obtusely angular, not defined by a marginal sinus, 

 and apparently never quite as long as the margin below ; posterior wing 

 decidedly longer than the other, but not as long as the posterior margin, 

 compressed, acutely angular at the extremity, and defined by a rather deep, 

 broadly-rounded sinus ; posterior basal margin rather narrowly rounded ;* 

 basal margin forming a broad semiovate curve, being more prominent poste- 

 riorly, and rounding up obliquely anteriorly into the regularly-rounded front; 

 beak moderately oblique, scarcely rising above the hinge-margin, and placed 



