346 
PALAEONTOLOGY OF NEW YORK. 
Derbya ruginosa, sp. nov. 
PE ATE XI A, FIGS. 25-27. 
Shell subelliptical in outline. Hinge-line short, its length being about two- 
thirds the greatest diameter of the shell. Pedicle-valve shallow; cardinal 
area moderately high, its lateral slopes being slightly more than one-half the 
length of its base; apex scarcely prominent; surface depressed or flat in the 
umbonal region, becoming irregularly concave anteriorly. Entire valve very 
irregular in growth, with concentric ridges and furrows. Brachial valve 
very convex; apex depressed, but the umbonal region gibbous, the greatest 
convexity being reached at the center of the valve. This valve is also of 
irregular growth, though the irregularities are not so strongly developed as 
on the opposite valve. The original specimen is an internal cast in chert 
to which portions of the inner laminae of the shell adhere. There are evi¬ 
dences of a flabellate muscular scar on the pedicle-valve and a short ovate 
muscular area in the brachial valve. 
The traces of the surface striae preserved show them to have been very 
fine and numerous. 
Keokuk limestone. Nev) Providence, Indiana. 
This species is similar in some general respects to Derbya Broadheadi, but 
differs in its narrower and lower cardinal area, less convex umbo on the brachial 
valve and in the absence of a median sinus on this valve. It may be compared 
with the Streptorhynchus crenistria, var. senilis, Phillips (Davidson), from the lower 
Carboniferous of Great Britain. 
Derbya? costatula, sp. nov. 
PLATE XIb. FIGS. 16,17. 
Shell small, outline semi-oval. Hinge-line nearly equal to the greatest diameter 
of the valve. Cardinal area moderately high, with a prominent deltidium 
very wide at the base. Pedicle-valve with an elevated beak from which the 
surface slopes to the margins with a tendency to irregular growth. Brachial 
valve faintly depressed at the umbo, but otherwise pretty regularly convex, 
