GO UNITED STATES GEOLOGICAL SURVEY OF TEE TERRITORIES. 



liion'i'ji in ii s uudafoundiis, M. & H. 



Plate 3, figs. 2, a, b. 

 Inoceramus tindabuvdue, Meek ami Hayden (1S62), Proceed. Acad. Nat. Sei. Pbilad., XIV, 26. 



Shell obliquely rhombic-subovate or subquadrate, gibbous; anterior 

 side very short, and rounding obliquely downward into the base; posterior 

 basal extremity prominently rounded; posterior margin broadly rounded or 

 subtruncated ; dorsal and anterior margin diverging from the beaks at an 

 angle of about 90° ; hinge short ; beaks moderately prominent and nearly 

 terminal, that of the left valve rather strongly incurved and directed obliquely 

 forward, while in the right it is straighter and less elevated ; umbonal axis 

 ranging at an angle of about 70° to the hinge-line. Surface of both valves 

 (in the condition of casts) ornamented by regular, strong, subangular, con- 

 centric undulations, separated by wider, rounded depressions. 



Height from the most prominent part of the base to the hinge-margin, 

 2.90 inches ; height to top of umbo, 3.3(3 inches ; convexity of left valve, 1.84 

 inches. 



The strong, subangular undulations, and gibbous, obliquely-subrhombic 

 outline of this species, will readily distinguish it from all the other known 

 forms of the genus in our rocks; and I know of no foreign species nearly 

 enough related to it to render a comparison necessary. Both of its valves 

 are quite convex, but the left one is rather decidedly more so than the right. 

 It seems to si and, as it were, almost exactly intermediate in its characters 

 between the typical Inoceramus and Catillus groups. In order that this may 

 be the more clearly understood, however, it should be explained that our 

 figure 2 a of this species on pi. 3, is not arranged with the hinge-line exactly 

 horizontal; the dorsal outline not being parallel to the hinge, which ranges 

 more obliquely forward and downward. If the figure were turned so as to 

 bring the beak farther upward, and the longer diameter of the valve more 

 nearly (hut not exactly) vertical, the hinge-line would then be horizontal. 



One of the specimens of the right valve has its beak so nearly terminal, 

 and its anterior margin below it so straightened in outline, as to present much 

 the appearance of a typical Inoceramus ; but as the others, as well as the left 

 valve we have figured, have the anterior margin more prominent, it is prob- 



