INVERTEBRATE PALEONTOLOGY. 47 



shell very distinctly inequivalve. The lateral curvature of the beak of the 

 left valve, together with its general form, give it much the appearance of some 

 species of Exogyra, as viewed from the inner side. Its aperture is trans- 

 versely oval, the height being to the length about as four to five. Remaining 

 portions of the shell show it to have been quite thick about the beak, and, as 

 in other species, distinctly fibrous. 



Specifically, this shell differs from the last in being much more 

 depressed, and in having its left beak considerably less elevated, and directed 

 much more obliquely forward, as will be seen by comparing fig. 3, a, pi. 5, 

 with fig. 2, b, pi. 4. It probably belongs, however, to the Volviceramus group. 



Locality and position. — Chippewa Point, near Fort Benton, on the 

 Upper Missouri ; from the same horizon as the last. 



Subgenus CATILLUS, Brong. 

 I ii o c e r a m u s pertenuis, M. & H. 



Plate 37, figs. 3, a, b; and plate 38, figs. 3, a, 6. 



Inoceramus ventricosus, Meek and Hayden (May, 1856), Proceed. Acad. Nat. Sci.Philad., VIII, 87 (not Sow- 



erby). 

 Inoceramus pertenuis, Meek and Hayden (Nov., 1856), ib., 276. 



Shell equivalve, or very nearly so, broad-oval or subcircular, very gib- 

 bous, apparently thin ; anterior side short, rounded or curving obliquely back- 

 ward and downward from the beaks ; base forming a broad curve, the most 

 prominent part of which is usually a little behind the middle ; posterior side 

 longer and higher than the anterior, very broadly rounded or subtruneate , 

 hinge-line of moderate length, straight, and forming an angle of about 50° 

 with the umbonal axis. Beaks tumid, rather obtuse, rising somewhat above 

 the hinge, and located nearly over the anterior extremity, often constricted 

 by a distinct sulcus around the umbonal region. Surface having a few irreg- 

 ular, obscure, concentric undulations, and marked by fine, regular, subimbri- 

 cating lines of growth. 



Length, 4.30 inches; height, 3.40 inches; convexity, 2.20 inches. 



The ventricose, cordate form of this species, and the peculiar tumid and 

 constricted character of its beaks, will serve to distinguish it from all the 

 other forms known in the Upper Missouri rocks that bear any general resem- 

 blance to it. In some respects, certain varieties of it remind one of I. multi- 

 plicatus, Stoliczka (Palseont. Indica, III, 406, pi. xxviii, figs. 1, la); but its 



