34 



FAM. CRASSATELLIDJE. 

 GENUS CRASSATELLA, Lam. 

 CRASSATELLA SUBQUADRATA, n. sp. 



Plate 11, fig. 12. 



Shell small, subquadrangular in outline, with thin, compressed valves 

 and not prominent beaks, which are situated within the anterior third 

 of the length ; length of the shell exceeding the height ; basal margin 

 broadly rounded ; anterior end short and narrowly rounded ; posterior 

 end broad and rounded truncate ; posterior hinge-line straight and sub- 

 parallel to the basal margin. Surface of the shell unknown. Muscular 

 impressions small; the posterior one situated close beneath the extremity 

 of the hinge-margin, and the anterior scar subinarginal. Pallia! line 

 not distinctly marked; margin of the valves crenulate. 



The only specimen of this genus recognized among the collections 

 from the Black Hills is a small internal cast, answering to the above 

 description, and apparently a true Orassatella. Although the extremity 

 of the hinge-plate has not been seen, the cast has all the features usually 

 possessed by casts of the typical forms of the genus. 



Formation and locality. In Cretacous beds on the Cheyenne River ; 

 probably from the Fort Pierre' group. 



CTPEINID^E. 



GENUS SPILERIOLA. 

 SPH^ERIOLA TRANSVERSA, n. sp. 



Plate 10, figs. 14-16. 



Shell of rather large size, subsphaeroidal, a little transverse, or longer 

 than high, exclusive of the projection of the beaks, the outline of the 

 margin of the valve being slightly oblate or transversely broad-oval, and 

 nearly symmetrical. Valves highly inflated on the central area and 

 umbones, with strong, highly-elevated, strongly-incurved, subcentral 

 beaks, which are slightly curved forward and subangular on the um- 

 bonal ridges. 



Surface of the shell marked by sublamellose, concentric lines of growth , 

 which form strong distant undulations in some cases toward the ex- 

 tremities of the shell. 



Internally the muscular impressions are large and subcircular, and 

 the pallial line simple and entire. The cardinal teeth, as obtained by a 

 gutta-percha mold from a natural cast, consist of two tuberculiform 

 teeth beneath the beak, and the laterals are represented by a strong 

 oblique fold or ridge passing from near the beak to within a short dis- 



