178 CRETACEOUS PERIOD. 



Surface marked by moderately strong concentric plications, and also 

 l)y fine concentric lines of growth. 



The dimensions of tliis shell differ much with age in their relative 

 pro23ortions ; the older ones being proportionally much more elongate than 

 the young ones. This is due to the more rapid growth of the shell in and 

 near the postero-lateral region than elsewhere. The long diameter of one 

 of our examples is a little more than six centimeters ; the short diameter 

 about tlii'ee and a half centimeters. 



This is perhaps one of the' most widely -distiibuted species of the genus 

 in the Cretaceous strata of the United States, and also perhaps one of the 

 most variable. This, together with the fact that no species of Inoceramus 

 possesses very salient specific characters, often renders its identification a 

 matter of some difficulty, especially if the specimens are imperfect. There 

 seems, however, to be very little reason for doubting that the forms under 

 examination belong to the species /. prohlematicus, as originally described by 

 Schlotheim. 



Position and locality. — Stratca of the Cretaceous period: southeast of 

 Paria, Utah ; and also upon the West Fork of Paiia Creek, Utah. 



Inoceramus fragilis Hall and Meek. 

 Plato XV, fig. 3 a. 

 Inoceramus fragilis Hall and Meek, 1856, Mem. Am. Acad. Arts & Sci., n. s., v, 388. 



Shell of medium size, obliquely ovate in marginal outline, somewhat 

 inflated ; beaks small, pointed, turned forward ; hinge-line rather long ; 

 surface marked by concentric undulations and fine lines of growth, but the 

 shell has a somewhat smooth aspect. 



The collections contain only a single example of this species, which 

 has been recognized by Mr. Meek as identical with /. fragilis. 



Position and locality. — Strata of the Cretaceous period; southeast of 



Paria, Utah. 



Inoceramus flaccidus White. 



PL-ite XVI, fig. 1 a and h. 



Shell large, irregularly subovate in marginal outline, exclusive of the 

 ears; valves subequal, not much inflated; -udng moderately large, well defined 

 at its inner side by an auricular fun-ow ; hinge-line not very long, nearly 



