56 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



Locality and posit ton. — Yellowstone River, 150 miles above its mouth; 

 from the upper part of the Fort Pierre group; also, from the Fox Hills 

 group, on Deer Creek, a tributary of the North Platte. I think I have also 

 seen the same form from the Cretaceous of Alabama and New Jersey. 



Inoceramus B a I c Ii i i , M. & H. 



Plate 15. figs. 1. «. l>. 

 Inocei-amns Balchii, Meek and Haydeu (May, 1860), Proceed. Acad. Nat. Sei. Philad., XII, 180. 



Shell (left valve) large, subquadrate or broad oblong-oval, much com- 

 pressed; anterior side truncated obliquely forward above at an angle of about 

 115° with the hinge, rounding into the base below; ventral margin forming, 

 a broad curve, the most prominent part being behind the middle; posterior 

 side longer and wider than the other, broadly rounded (sometimes subtrun- 

 cated above); hinge-line rather long, forming an angle of about (J0° with the 

 umbonal axis; beaks narrow, rising a little above the hinge, scarcely incurved, 

 located about half-way between the middle and the most prominent part of 

 the anterior border. Surface ornamented by very slightly-elevated, broadly- 

 rounded, rather irregular undulations, which become entirely obsolete on large 

 specimens, below the middle, and on the posterior and lower anterior regions. 



Of the finer surface-markings of this shell, if there were any, we know 

 nothing; the only specimen yet found being an internal ca*t, with portions of 

 the inner laminated layer remaining. It will be readily distinguished from 

 I. Vanuxemi by its less distinct and more irregular as well as broader undu- 

 lations. Its beaks are also much more prominent and oblique, and placed 

 farther forward, while the general form of the shell is more oval transversely, 

 and its anterior side more distinctly truncated. Its proportionally moie com- 

 pressed form, more obscure and less regular undulations, longer hinge, and 

 truncated anterior, will prevent it from being confounded with I. Nebrascensis, 

 Owen. 



The only specimen yet found of this species is not in a condition to show 

 clearly the exact form of the adult shell; but the curve of the undulations in 

 the region of the beaks shows that when it had attained a length of 3.40 

 inches, its height was 2.90 inches. The specific name was given in honor of 

 Lieut, G. T. Balch, of the United States Ordnance, to whom we are indebted 

 for the typical specimen of the species. 



Lorn/ili/ and position. — Same as preceding. 



