348 
PALAEONTOLOGY OF NEW YORK. 
Derbya Bennetti, sp. nov. 
PLATE XIA, FIGS. 34-39. 
Shell sub trihedral in general aspect, quite irregular in its growth. Hinge¬ 
line short, its extremities on both valves being auriculate. Pedicle-valve 
much the more irregular in growth, sometimes retaining the scar of attach¬ 
ment at its apex. Cardinal area unusually high, narrow, erect or slightly 
incurved, and frequently distorted; delthyrium curved. General surface of 
the valve depressed-convex in the middle, sometimes rapidly sloping in all 
directions, at others concave in the umbonal region; as a rule very unsym- 
metrical. The brachial valve is deep, more regularly convex and has a full 
rounded umbo and a conspicuous median sinus. On the interior the pedicle- 
valve bears an extremely high median septum which is united with the den¬ 
tal ridges near the apex. The cardinal process is high, erect and deeply 
bilobed, each of its apophyses being strongly grooved on its posterior face. 
Other internal characters unknown. 
The surface of both valves is covered by fine, elevated, thread-like striae 
increasing very slowly by intercalation. The edges of these striae bear nu¬ 
merous minute asperites which may be due to the crossing of fine concentric 
lines. Irregular lines and wrinkles of growth are abundant near the margins. 
Upper Coal Measures. Near Kansas City, Missouri. 
Derbya cymbula, sp. nov. 
PLATE XIB, FIGS. 2, 3. 
Shell large ; marginal outline transversely subelliptical. Hinge-line straight, 
its length being about two-thirds the greatest diameter of the shell On the 
pedicle-valve the cardinal area is high, its base being one-third longer than 
its sides, and it may be somewhat unsymmetrical from distortion. Its surface 
is finely striated both longitudinally and transversely, and is divided into an 
outer and inner portion by two lines diverging from the apex and meeting 
the hinge-line half-way between its extremities and the edges of the deltidium. 
Deltidium broad at the base, rapidly narrowing for one-third its length, 
