INVERTEBRATE PALAEONTOLOGY. ].']«) 



tions of the last -described species; that is, that the only specimen known 

 is not in a condition to warrant any satisfactory conclusions on that point. 

 Consequently, it is only referred to the same genus with great doubt, merely 

 provisionally,' until better specimens can be procured for study and com- 

 parison. 



The specific name is given in honor of General G. K. Warren, of the 

 United States Topographical Engineers, among whose collections (obtained 

 during one of his exploring expeditions) the type-specimen was found. I 

 have had it under consideration for many years, without attempting to 

 describe it, hoping that better specimens, giving some knowledge of its 

 hinge-characters, might be found. 



Locality and position. — Near the eastern base of the Back Hills ; from 

 the Fox Hills group of the Upper Missouri Cretaceous series. 



Spliicriola ! endolrachys, Meek. 

 Plate 29, fig. 2. 



Shell rather thin, orbicular, ventricose, about as high as wide ; anterior, 

 posterioi-, and basal margins regularly rounded; dorsal margins sloping and 

 rounding off nearly equally each way into the lateral; beaks elevated, central, 

 and incurved, with but slight obliquity ; muscular impressions very shallow, 

 rather large, the posterior ovate, and the anterior trigonal-subovate ; interior 

 surface within the pallial line roughened by irregular projecting asperities 

 that leave the surface of internal casts distinctly pitted ; external surface 

 marked by rather distinct, irregular, concentric furrows and striae. 



Length, 1.50 inches; height, 1.45 inches; convexity, about 1.08 inches. 



This species will be readily distinguished from the last, which it most 

 nearly resembles in form, by its rather decidedly thinner shell, less gibbous 

 valves; and particularly by the peculiar rough character of its interior within 

 the pallial line. This latter character is strongly marked in all three of the 

 specimens seen ; while no traces of anything of the kind are observed in 

 either of the previously-described species. Like those shells, this is merely 

 placed in this genus provisionally, nothing being known of its hinge- 

 characters, excepting that it has no lateral teeth. 



Locality and position. — Ninety miles below Fort Benton, on the Missouri; 

 liom Cretaceous beds holding a position in the very upper part of the Fox 

 Hills group. 



