42 BRACHIOPODS. 



from the beak to the front, rounding down rather strongly on 

 each side to the ears, which are not abruptly separated from the 

 swell of the umbo ; central region with a shallow, narrow mesial 

 sinus, extending from the front about two thirds of the way to 

 the beak ; umbonal region not very prominent; beak small, 

 incurved, and passing but slightly beyond the hinge-margin ; 

 interior unknown. Dorsal valve distinctly concave but nearly 

 flat over a large portion of the central region, and strongly 

 curving up at the front and lateral margins, usually with a 

 slight mesial ridge corresponding to the sinus of the other 

 valve. Interior with a rugose ridge extending around near 

 the front and lateral margins, so as to present a somewhat 

 geniculated appearance, not seen on the outside ; cardinal pro- 

 cess stout, apparently rather short, and bifid, its base forming 

 a short stout ridge, which soon becomes obsolete near the 

 muscular scars, from between which a narrow ridge extends 

 forward two-thirds to three-fourths the length of the valve, 

 becoming sharply elevated and thin at the end ; scars of 

 adductor muscles elongated, parallel and rugose ; reniform 

 scars rather broad, and somewhat roughened by a few irregu- 

 lar wart-like prominences; space between the reniform scars 

 and the mesial ridge flat, and without any traces of the sub- 

 conical prominences seen in P. giganteus ; posterior lateral 

 regions irregularly pitted or punctured. 



Surface of both valves ornamented by numerous, rather 

 coarse, often waved or flexuous, striae, or small obscure costre, 

 that increase by intercalation and division, all sometimes be- 

 coming nearly obsolete on or near the ears ; fine concentric 

 stripe are also seen on all parts of the surface, and over the 

 visceral region very obscure traces of small concentric wrinkles 

 likewise occur. On the ventral valve, bases of small spines 

 are seen irregularly scattered, being most numerous, largest 

 and closely arranged on the ears and along the hinge-margin, 

 ^o spines occur on the dorsal valve, but little pits seem to 

 occupy their places. ( Meek.) 



Horizon and localities. Lower Carboniferous, Keokuk 

 limestone : Ste. Genevieve, St. Francisville (Clark county). 



