246 PROCEEDINGS OP THE ACADEMY OF 



Genus MODIOLOPSIS, Hall, 1847. 



MODIOLOPSIS PEROVATA, M. & W. 



Shell longitudinally ovate, the widest part being a little behind the middle, 

 compressed, very thin, extremely inequilateral and oblique; posterior side 

 compressed, cuneate, regularly rounded in outline ; anterior side very short, 

 more narrowly rounded than the posterior margin. Dorsal outline forming 

 a broad, nearly regular arch from the beaks into the posterior border ; base 

 oblique, and somewhat straightened just in front of the middle, and round- 

 ing up towards the extremities. Beaks compressed, scarcely projecting be- 

 yond the rounded anterior outline, and placed directly over the anterior ex- 

 tremity. Surface marked with regular concentric striae, and small, irregular 

 furrows. Anterior muscular impression oval, distinct, located close to the 

 margin, under the beak. 



Length, 1-92 inches ; height, 1-18 inches; convexity, (MO inch. 



This species has much the general appearance of Modiolopsis concentrica, 

 (Hall, Geol. Fourth Dist. p. 19b', fig. 9,) but differs in having its anterior out- 

 line rounded, instead of protuberant and subangular in outline. Its margin 

 is also more prominent in the antero-ventral region, and without "a longitu- 

 dinal impression directly below the beaks." 



Locality and position. White Sulphur Springs, Delaware County, Ohio. 

 Hamilton Group, of Devonian series. 



Genus PLEUROPHORUS, King, 1844. 

 Pleurophokus subcostatus. 



Shell elongate-oblong, moderately convex ; umbonal ridges the most convex 

 part of the valves, and extending obliquely from the beaks towards the pos- 

 tero-basal margin ; anteror ventral region somewhat compressed ; basal and 

 cardinal margins very nearly straight and subparallel, the former being usu- 

 ally somewhat sinuous or arcuate along the middle ; extremities rather nar- 

 rowly rounded, the posterior being generally a little wider than the other, 

 and sometimes faintly subtruncate obliquely. Hinge line long and nearly 

 straight ; posterior lateral tooth of each valve elongated parallel to the hinge 

 margin, very remote from the cardinal teeth, and extending back a little be- 

 yond the posterior muscular impression. Beaks depressed upon a line with 

 the dorsal margin, small, somewhat compressed, and placed about one-ninth 

 the entire length of the shell behind the anterior margin. Scar of the ante- 

 rior adductor muscle deep, trigonal-subovate, pointed above, and strongly de- 

 fined by the prominent vertical ridge just behind it ; those of the pedal 

 muscles small, nearly marginal, and located directly over the anterior ad- 

 ductors ; posterior adductor scars larger and more shallow than the anterior, 

 subquadr, te in outline, and placed close up under the posterior hinge teeth. 

 Pallial impression well defined. Surface of casts showing traces of a few ob- 

 scure concentric markings, crossed on the postero-dorsal region by traces of 

 about three equal obscure radiating costse. Exterior surface and cardinal 

 teeth unknown. 



Length of a medium sized specimen, (internal cast,) 88 inch ; height of 

 do., 0-37 inch ; convexity, 0-26 inch. Some larger specimens of same pro- 

 portion, measure 1*33 inches in length. 



This species is apparently related to P. costatus, of Brown (sp.), some vari- 

 eties of which, as figured by Prof. King <Monogr. Permian Fossils, England, 

 pi. xv.) it closely resembles, at any rate, so far as can be determined from the 

 comparison of internal casts. Yet it evidently differs from that species, in 

 having the scars of its pedal muscles nearly marginal, and placed directly 

 over those of the anterior adductors, instead of partly behind them upon the 

 internal ridge. This ridge is also stronger in our species ; while the basal 

 margins of its valves are likewise more sinuous. 



[Dec. 



