350 PALEONTOLOGY OF NEW YORK. 



Surface covered by sharply defined, sub-equal radiating striae, which in- 

 crease by implantation. The grooves between these striae are deep, and 

 both striae and grooves are crossed by fine concentric lines, which on the 

 former produced a series of sharp asperities. Interior, with the exception 

 of the median septum in the pedicle-valve, unknown. 



Upper Coal Measures. Near Kansas City, Missouri. 

 There are many points of similarity in the Orthis Kaskaskiensis, McChesney, 

 from the Kaskaskia limestone, Derbya cymbula and the species under consid- 

 eration. All have the same general aspect. In 0. Kaskaskiensis the brachial 

 valve is most convex at the umbo, the pedicle-valve generally concave and the 

 hino-e-line equal to the greatest diameter of the shell ; in Derbya affinis the 

 brachial valve also has its greatest convexity at the umbo, but the hinge-line 

 is considerably shorter than in McChesney's species, and there is a notable 

 difference in the character of the surfoce striae ; while in Derbya cymbula the 

 convexity of the brachial valve is greatest at its center, the hinge-line very 

 short and the pedicle-valve concave or depressed only over the pallial region. 



Derbya (?) biloba, sp. nov. 



PLATE XI, FIGS. 4, 5. 



Shell small, obcordate in outline. Hinge-line short and straight, its length 

 being considerably less than one-half the width of the shell. On the pedicle- 

 valve the cardinal area is moderately high and slightly arched backward ; 

 delthyrium covered. The surface of the valve is somewhat depressed or 

 flattened over the pallial region. The brachial is deeper and more convex ; 

 the umbo is full but not elevated, and just in front of the apex there begins 

 a broad and conspicuous sinus which widens rapidly and renders the shell 

 bilobate on its anterior margin. 



Surface covered with numerous fine radiating striae. Interior unknown. 



Upper Coal Measures. Winterset, Iowa. 



