OEETACEOUS FOSSILS. 401 



strong, irregular concentric undulations, with finer ones in the broader 

 interspaces, and crossed by indistinct radiating lines on internal casts. 

 The fibrous coating, where preserved, appears to have been very thin on 

 the younger portions of the shell, but toward the outer margin, in larger 

 individuals, it attains a thickness of a sixteenth of an inch or more. None 

 of our specimens are in a condition to show the surface of the fibrous coat- 

 ing, consequently we have not seen on them the fine concentric linings 

 characteristic of the species. 



There is, perhaps, no better marked, or more distinct, species than this 

 among the Inocerami of the Black Hills region. It is most nearly related 

 to the forms referred to I. bambini, but is easily distinguished by the 

 larger and more prominent beaks, the inflated valves, and broad, abrupt 

 anterior end, and the abrupt, concave posterior cardinal slopes, as well as 

 the more nearly parallel basal and cardinal margins. 



Formation and locality. — At the top of the gray shales of the Fort Pierre' 

 Group, on the Cheyenne River, near Rapid Creek, Black Hills, Dakota 



Among the Inoceramus-like shells in the collection are two species, 

 which are characterized by a strong and broad internal rib on each valve, 

 extending obliquely backward from behind the beaks along the posterior 

 umbonal slope to near the postero-basal angle of the shell, leaving upon the 

 internal casts a concave depression or sulcus of greater or less depth, and 

 increasing in width and depth from the beaks outward. This rib does not 

 affect the outer surface, nor produce any external feature on the shell, being 

 exclusively confined to the inner nacreous layers, and formed by the addi- 

 tion of successive coatings on the inner surface only and within the margin 

 of the valve, so that it is seen only on the interior of separated valves, on 

 internal casts, or on exfoliated specimens. From the interior of valves it is 

 readily removed, even from the outer nacreous layers, as the feature is sel- 

 dom developed at the edge of the shell during growth, but at a point some 

 distance within the margin. It can sometimes also be detected on examples 

 where the pearly layers are nearly all preserved, showing the margins of 

 the depression faintly defined through the partially transparent substance. 

 In such cases, however, it is only detected upon close observation. 



All individuals of species possessing this feature do not have it devel- 



26 B H 



