492 NORTH AMERICAN INDEX FOSSILS. 



353. A. oklahomaensis Beede. Permic. 

 Differs from A. occidentalis in its frequently smaller angle at the 



beak, and in the more pronounced concentric lamellae. 



Upper Permic : Oklahoma (Whitehorse), Texas (Quarter- 

 master). 



XCII. Acanthopecten Girty. 



In surface characters distinguished from Aviculopecten by its 

 large, angular plications, and spinose margin and growth lines. 

 Carbonic. 



354. A. carboniferus (Stevens). (Fig. 659, a, b). Carbonic. 

 Small ; right valve less spinose, and less strongly plicate than 



left. Type of the genus. 



Distributed widely in the Coal Measures from West Virginia to 

 Colorado and New Mexico. 



XCIII. Euchondria Meek. 

 Like Aviculopecten but with an unsymmetrical resiliifer beneath 

 the beak and with numerous pits on the two 



sides of it unequal in number and shape. 



Carbonic. 



355. E. neglecta Geinitz. (Fig. 660.) 



Carbonic. 

 Small, with concentric striae only on body 



Fir, 660. Euchondria of shell » but ribs on ears - T yP e of S enus - 



neglecta, right valve, X2; Illinois, Iowa, Kansas, Nebraska, Missouri. 



hinge greatly magnified, 



showing oblique cartilage XCIV. PTERINOPECTEN Hall. 



and smaller lateral pits. Differs f Aviculopecten ' in the longer 



/ pi Til "IT \ -L O 



hinge line, and ears not well defined ; from 

 Lyriopecten in its distinctly angular anterior ear. Devonic. 



356. P. exfoliatus Hall. (Fig. 661, d.) Devonic. 

 Subrhomboidal. Left valve very convex ; right less convex. 



Hinge line longer than shell below. Umbo prominent. Surface 

 of left valve marked with strong rays alternating with one or two 

 finer ones ; rays on right valve less strong. 

 Marcellus : New York. 



357. P. intermedius Hall. (Fig. 661, c.) Devonic. 

 Differs from P. exfoliatus in its greater obliquity, smaller umbo, 



comparatively smaller posterior and larger anterior ear. Surface 

 rays more thread-like. 



