284 REPORT UNITED STATES GEOLOGICAL SURVEY. 



examples. The true marginal outline of that large example is also in part 

 obscure in the specimen, and probably does not differ so much from that 

 of the others as Mr. Meek supposed. The angularity of the umbonal ridge 

 of this large specimen is evidently natural and not accidental, as appears 

 more plainly upon the gutta-percha cast which I have made, than upon 

 the specimen ', and the umbonal ridge of all the other examples is marked 

 in a similar manner, which really consists of a radiating rib in the case 

 of all of them. It is true that no radiating ribs are to be seen upon the 

 large example in front of the umbonal one, but they are distinct only 

 upon the small examples, which are evidently young, and obscure upon 

 the larger intermediate sizes. They seem to have become obsolete, and 

 finally obliterated with age. 



Genus mOCEEAMUS Sowerby. 



iNOCEEAirUS HOWELLI White. 



Plate 4, figs. 1 a, h, and c. 

 Inooeramus howelli White, 1876, Powell's Rep. Geol. Uinta Mts. p. 114. 



Shell of medium size, obliquely and irregularly suboval in marginal 

 outline, the vertical diameter being greater than the transverse ; the left 

 valve greater than the other, but both of them have considerable con- 

 vexity; beaks narrowed, prominent, the prominence of the left one 

 greater than that of the other ; both of them elevated above the hinge- 

 line, and also curving forward beyond the front of the shell ; front more 

 or less flattened, extending almost straight downward from the front end 

 of the hinge, with which it forms nearly a right, or slightly obtuse angle ; 

 antero-basal margin abruptly rounded to the base ; basal margin short ; 

 postero-basal margin extending obliquely upward to the posterior ex- 

 tremity, straightened or slightly emarginate ; posterior extremity abruptly 

 rounded to meet the almost straight postero-dorsal margin. 



Between the axis of the body of the shell and the postero-dorsal mar- 

 gin there is upon each valve a rather broad, shallow, but more or less 

 distinct furrow or depression, extending from the umbonal region to the 

 postero-basal margin and ending in the emargination before mentioned. 

 There is also a distinct alation upon each valve, separated from the body 

 portion by a tolerably well-defined auricular furrow. 



Surface marked by the ordinary lines of growth and also by the mod- 

 erately distinct concentric folds, but the surface has a rather smoother 

 aspect than is common with species of this genus. 



Height of an average-sized example, from base to beaks, 7^ centi- 

 meters 5 greatest breadth, which is near the base, 5 centimeters ; length 

 of hinge, 37 millimeters. 



This shell has the general aspect of /. fragilis Hall & Meek, but dif- 

 fers from it in possessing the shallow radiating furrow upon the body 

 of each valve, and also in having a distinct posterior ear, separated from 

 the body of the valve by an auricular furrow. It also resembles an ex- 

 ample of I. striatus Mantell, in the cabinet of the Smithsonian Institu- 

 tion, from Saxonj^, but the beaks of our species are more elevated and 

 turned more forward than they are in that species. J. striatus is also 

 without the shallow radiating furrow belore mentioned. It differs from 

 J. flaceidus White (Expl. and Surv. West of 100th Merid. vol. iv, pt. 1, 

 p. 178, pi. xvi, figs. 1 a and h) in its smaller size, its smoother surface, 

 and more gibbous valves, that species being considerably flattened, and 

 coarsely and extravagantly wrinkled. 



