348 PALAEONTOLOGY OF ILLINOIS. 



usually somewhat sinuous or arcuate along the middle ; extremi- 

 ties rather narrowly 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 mar- 

 gin, very remote from the cardinal teeth, and extending back 

 a little beyond the posterior muscular impression. Beaks de- 

 pressed 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 anterior 

 adductor muscle deep, trigonal-subovate, pointed above, and 

 strongly defined by the prominent vertical ridge just behind it; 

 those of the pedal muscles small, nearly marginal, and locateH 

 directly over the anterior adductors; posterior adductor scars 

 larger and more shallow than the anterior, subquadrate in out- 

 line, and placed close up under the posterior hinge teeth. Pal- 

 lial impression well defined. Surface of casts showing traces 

 of a few obscure concentric markings, crossed on the postero- 

 dorsal region by traces of about three equal obscure radiating 

 costge. Exterior surface and cardinal teeth unknown. Length 

 of a medium sized specimen (internal cast), 0.88 inch; height 

 of do., 0.37 inch; convexity, 0.26 inch. Some larger speci- 

 mens, of same proportions, 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 inter- 

 nal ridge. This ridge is also stronger in our species, while the basal margins 

 of its valves are likewise more sinuous. 



This shell is also scarcely distinguishable from a form that has been regarded 

 as a variety of P. subcuneatus, Meek and Hayden, in the Permian and Permo- 

 carboniferous beds of Kansas. Whether specifically identical with this Kansas 

 shell (the supposed variety of P. subcuneatus), may admit of some doubt; but 

 it certainly differs materially from the typical form of P. subcuneatus, not only 



