452 University of Kansas Geological Survey. 



persistent throughout the Fort Benton area of Kansas. The 

 fossil exists in all of the Fort Benton limestone layers and in 

 the intercalating shale beds. Its abundance is one of the most 

 convenient means of distinguishing the Fort Benton limestone 

 from the Fort Hays of the Niobrara, since it is not abundant, 

 if it occurs at all, in the Niobrara rocks of Kansas. From other 

 states it is reported as being an abundant and characteristic 

 fossil of the Niobrara. Aside from the Ostrea conge sta, it is the 

 most abundant species of the Colorado formation in Kansas. 



Inoceramtis dimidius White. Plate xcviii, figs. 5, 6. 



Tnoceramus dimidius White, 1874, Expl. and Surv. West 100th Mer., Prelim. 

 Rep. Inv. Foss., p. 25; 1876, U. S. Geol. and Geog. Surv. West of the 100th 

 Mer., vol. iv, p. 181, pi. xvi, fif. 2a, b, c and d; Stanton, 1892, Bull. U. S. 

 Geol. Surv., p. 78, pi. x, ff. 5 and 6. 



Description : " Shell very small for one of this genus, inflated, 

 sometimes much so, obliquely subovate in outline ; valves sub- 

 equal, the left one being a very little more capacious than the 

 other ; test thin ; beaks small, prominent, acute, incurving, and 

 pointing a very little forward ; hinge line straight or nearly so, 

 rather short. 



" Surface marked by more or less strong and more or less regu- 

 lar concentric folds or undulations. In some cases these un- 

 dulations continued to be formed only until the shell attained 

 about half its full size, when they ceased, the remainder of the 

 surface being marked only by ordinary concentric lines of 

 growth. This irregularity in the formation of concentric folds 

 is sometimes connected with considerable distortion of the usual 

 symmetry of the shell. 



" The long diameter of an average example, from the umbo to 

 the postero-ventral margin, twenty-six mm. ; greatest breadth, 

 eighteen mm. ; thickness, sixteen mm. This species is especially 

 distinguished by its small size. Its other more conspicuous 

 specific characters are the small but prominent and pointed 

 beaks, and subequal valves. 



"From the young of I. labiatus, the valves of which are sub- 

 equal, it differs in the character of the beaks just mentioned, 



