78 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. 106. 



always easily recognized by its obliquely elongate outline and by the 

 character of the surface markings. 



It is unfortunate that the law of priority compels us to drop the name 

 that has for many years been applied to this species by all American 

 writers who have referred to it, and it is the more unfortunate because 

 the species is so well known under that name as the most abundant 

 characteristic fossil of the Colorado formation. There is no doubt, how- 

 ever, that the American fossil belongs to the species to which European 

 paleontologists now apply the earliest name, Inoceramus labiatus. 



Locality and position. — The species is common in the Niobrara lime- 

 stone of Kansas, Nebraska, Colorado, and throughout the upper Mis- 

 souri river region. It is also very abundant in calcareous layers of 

 the Fort Benton shales wherever they occur in the same regions, and 

 it is found in the equivalent strata in Utah, New Mexico, Texas, North- 

 ern Mexico, etc. In Europe the species is said to be confined to the 

 Lower Turonian and in southern India it is found only in the Ootatoor 

 group which forms the base of the Upper Cretaceous section of that 

 region. 



Inoceramus dimidius White. 



PI. x, Figs. 5 and 6. 



Inoceramus dimidius White, 1874, Expl. and Snr. West 100th Meridian, Prelim. Rept. 

 Invert. Foss., p. 25; 1876, U. S. Geog. and Geol. Sur. West of the 100th Meridian, 

 vol. iv, p. 181, PI. 16, Figs 2a, b, c, and d. 



The revised description is as follows: 



" Shell very small for one of this genus, inflated, sometimes much 

 so, obliquely subovate in outline; valves subequal, the left one being a 

 very little more capacious than the other; test thin; beaks small, prom- 

 inent, acute, incurving, and pointing a very little forward; hinge-line 

 straight or nearly so, rather short. 



u Surface marked by more or less regular and more or less strong 

 concentric folds or undulations. In some cases these undulations con- 

 tinued to be formed only until the shell had 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 forma- 

 tion of concentric folds is sometimes connected with considerable dis- 

 tortion of the usual symmetry of the shell. 



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

 postero- ventral margin, 26 mm ; greatest breadth, 18 mm ; thickness,16 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 Lproblematicus, 

 the valves of which are also subequal, it differs in the character of the 

 beaks just mentioned, the much greater convexity of the valves, and 

 other evidences of mature growth. 



u The collections contain quite a large number of examples of this 



