350 
PALAEONTOLOGY 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 
hinge-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 surface 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, i, 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. Winter set, Iowa. 
