CRETACEOUS FOSSILS. 395 



dations in character of this kind, that to separate the form we have figured 

 as a variety or species would require that we should, in the same way, 

 separate almost every individual specimen under a different name." Taking 

 into consideration these extensive variations, we are inclined to consider the 

 different forms of this type as only varieties of the one species. Mr. Meek 

 appears, from his description and remarks, to have considered his I. balchii 

 as allied to I. vamixemi, and consequently as allied to I. proximus Tourney; 

 but from the character of the beak and that of the concentric undulations 

 we should think it more nearly related to this form. 



Formation and locality. — In Cretaceous beds of the Fort Pierre Group, 

 on the Cheyenne River, near Rapid Creek; also at the top of the gray 

 shales at the mouth of Rapid Creek, Black Hills. 



INOOERAMUS SIMPSONI. 



Plate 8, fig. 1. 



Inoceramus simpsoni Meet, Proc. Acacl. Nat. Sci. Phil. 1860, vol. 12, p. 312. 

 Jnoceramus simpsoni Meek, Rept. of Expl. across tlie Great Basin of Utah, p. 360, PI. 

 4, fig. 4. 



Shell very large, transversely and obliquely-ovate, with strongly con- 

 vex and slightly unequal valves, which are the most ventricose along the 

 antero-basal third of the shell, and gradually compressed toward the pos- 

 tero-cardinal margin. Right valve apparently the most convex on all the 

 specimens examined, and the greatest convexity a little more forward than 

 that of the left valve. Hinge line long and straight, extending from the 

 beaks to about the posterior third of the valve, where it gradually slopes to 

 the posterior extremity. Beaks of moderate size, not very prominent, and 

 but slightly projecting beyond the line of the hinge in comparison with 

 the size of the shell, situated near the anterior end of the shell, but not ter- 

 minal; anterior end round and full, much narrower than the posterior end 

 and gradually rounding into the basal line, which is broadly curved and 

 prolonged in the direction of the umbonal slope; posterior extremity 

 obliquely rounded from the end of the cardinal line to the postero-basal 

 margin 



Surface of the shell marked by numerous concentric undulations; 

 which gradually increase in width from the beaks as the shell increases in 



