OEETACEOUS FOSSILS. 391 



from the separated valves in the collection, usually more evenly convex 

 than the left, being destitute of the slight angularity along the umbonal 

 ridge. 



Formation and locality. — In Cretaceous strata referred to the Fort Benton 

 Group, on Beaver Creek, and on the eastern fork of the same, Black Hills. 



INOCEEAMUS ALTUS. 

 Plate D, fig. 11. 



Inoceramus altus Meek, Geol. Eept. U. S. Surv. Terr. 1871, p. 302. 

 Inoceramm altus Meek, Pal. U. S. Geol. Surv. Terr. 1876, p. 43, PI. 14, fig. 1. 



The specimens of this species present in the collection are all more or 

 less imperfect and fragmentary ; they are so strongly marked, however, that 

 they are very readily distinguished from any other form yet observed. The 

 shells are obliquely ovate exclusive of the alation of the hinge, which is 

 usually absent, and strongly convex, especially on the umbones and along 

 the umbonal ridge ; the beaks are only of moderate size, strongly incurved, 

 with a forward inclination, are pointed and terminal, and project somewhat 

 beyond the cardinal line ; anterior border short and nearly vertically 

 rounded to the umbones, extending along the valve to a little below the 

 middle, where it rounds into the basal margin ; base prolonged obliquely 

 backward in the direction of the umbonal ridge. 



Surface marked by small, but very distinct, concentric undulations, 

 which are remarkably uniform in size, being generally about an eighth of 

 an inch in width or a little more, and not sensibly increasing with the 

 increased growth of the shell. Substance of the shell, as preserved on the 

 specimens, highly nacreous. 



The example figured by the author, loc. cit., is of very much larger size 

 than is indicated by any of ours, and the undulations somewhat more dis- 

 tant, increasing in size toward the margin, which is not the case with those 

 under consideration. He also mentions radiating lines on the surface of 

 what he speaks of as a cast. Our specimens all preserve more or less of 

 the nacreous layers, and do not show radiating lines. 



Formation and locality. — In limestones of Lower Cretaceous, probably 

 of No. 2, as they are associated with I. perplexus and Scaphites wyomingensis< 

 M. & H., on the Belle Fourche, ten miles west of Crow Peak, Black Hills. 



