INVERTKBRATB PALEONTOLOGY. 61 



able thai the straightness of the anterior outline in the single specimen men- 

 tioned may be, to some extent, due to accidental distortion. 



Localiti/ and position. — Chippewa Point on the Missouri, near Fort lien- 

 ton ; from the Fort Benton group of the Upper Missouri Cretaceous series 

 Collected by Lieutenant Mullen. 



Inoc < ■ i- si ill ii s i ii c ii r v ii s , M & H 



Plate 12, li^s. 4, a, 6. 

 Inoceramus i item-run, Meek ami Haydeu (Nov., 1S5G), Proceed, Acad. Nat. Sci. Pbilad., VIII, 277. 



Shell subglobose, oval or ovate-cordiform, very gibbous, nearly or 

 quite equivalve; beaks oblique, small, placed a little back of the anterior 

 side, strongly incurved ; umbonal region of both valves very ventricose, and 

 considerably elevated above the hinge and points of the beaks. Surface 

 ornamented by concentric undulations, which become sharply elevated over 

 the most prominent portions of the umbones, but are less distinct, or nearly 

 obsolete, toward the base and extremities of the shell. 



Judging from the curve of the undulations near the beaks, young indi- 

 viduals of this species must have been nearly ovate in form ; the posterior side 

 being broader than the other. At this stage of its growth, the shell seems to 

 have been much less convex, and the beaks more nearly terminal ; but as it 

 advanced in age, it became rapidly more ventricose, and extended somewhat 

 in front of the beaks. The surface was probably marked by lines of growth, 

 but, owing to the exfoliation of the external fibrous layer, they are not pre- 

 served on our specimens; none of which are in a condition to give a very 

 clear idea of its general form. The peculiar, and equally-incurved character 

 of the beaks, together with the ventricose, elevated umbonal region, and 

 sharply-prominent undulations, when taken together, well serve to distinguish 

 it from any other species known from these rocks. 



I have placed this species here at the end of the group Catittus, merely 

 provisionally, the specimen being too imperfect to show its subgeneric 

 relations. 



Localiti/ and position. — Little Bear's Village, between Fort Pierre and 

 Fort Clark; in the Fort Pierre group, or formation No. 4 of the Upper 

 Missouri Cretaceous. 



