LOWER OAKBOXIFEROUS FOSSILS. 529 



hinge. The ears of the ventral valve are furnished with a bunch of spines. 

 The dorsal valve is similarly ornamented except that it does not appear to 

 be spinose and the radiating ridges become grooves. However, as the 

 convex side is usually presented to view, the surface seems a counterpart of 

 the other valve. Viewed from this side, two broad, strong, slightly diver- 

 gent grooves are seen near the hinge line, marking off the somewhat 

 upturned ears. 



PI. LXVIII, figs. 9a, 9b, show a type which I had thought to consti- 

 tute a distinct variety, but more careful comparison shows that this conclusion 

 was not warranted. 



P. cooperensis is said by Swallow to be common in the Chouteau lime- 

 stone of Cooper County, Missouri. I have studied a large series of speci- 

 mens from the Chouteau limestone of Stevens Fork, Missouri, and Chouteau 

 Springs, Cooper County, Missouri. This material agrees with Swallow's 

 description and is specifically identical with the specimens from the Yellow- 

 stone National Park. 



P. cooperensis proves to be a variable form. Many of the differences 

 noticed are without doubt due to differences in age, but a considerable 

 rana*e of variation can not be ascribed to this factor. The ventral valve is 

 sometimes low and flat, like that figured by Hall, 1 but often it is highly 

 arched. The dorsal valve is moderately concave, semielliptical ; beak 

 small, depressed. Often nearly plane at first, or sometimes concave, but 

 later becoming geniculate at the margin, and the geniculate portion is often 

 considerably prolonged. There is also a variation in size, some large shells 

 being depressed and immature in appearance, while much smaller ones are 

 highly arched and look fully grown. No reliance can be placed upon the 

 spines for specific identification. They are scattered over the surface, 

 sometimes irregularly, sometimes in rows, sometimes nearly absent. There 

 is often a bunch of strong spine bases on the rather small ears; but this, 

 too, is not a constant feature. 



The specific integrity of P. concentrica Hall, P. shwmardiana Hall, and 

 P. pyxidata Hall has often been questioned. As early as 1865 Winchell 2 

 stated the opinion that P. shumardiana and P. pyxidata are both synonyms 



i Geol. Surv. Iowa, Vol. I, Pt. II, PI. VII, fig. 2. 



2 Winchell, 1865: Proc. Acad. Nat. Sci. Philadelphia, \i. 115. 



MON XXXII, PT II 34 



