346 PALAEONTOLOGY OF NEW YORK. 



Derbta ruginosa, sp. nov. 



PLATE XIA, 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 Derhya 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 XlB. 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, 



