348 PALEONTOLOGY OF NEW YORK. 



Derbya Bennetti, sp. nov. 



PLATE XIA, FIGS. 34-39. 



Shell subtrihedral 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 XI B, 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, 



