246 



NORTH AMERICAN INDEX FOSSILS. 



at the spine bases. Very fine concentric growth lines present in 

 front and above, a few obscure transverse wrinkles. 



Kaskaskia of West Virginia, Kentucky, Ohio, Indiana, Missouri, 

 Illinois, Utah. 



152. P. semireticulatus (Martin). (Fig. 296.) 



Lower and Upper Carbonic. 



Large, much like P. costatus in form and character of plications, 

 but larger and with less distinct plications which are crossed by 

 regular concentric wrinkles in younger stages. Shallow mesial 

 sinus present giving the shell a bilobed appearance. 



Throughout the Carbonic of North America. 



153. P. Cora d'Orbigny. (Fig. 297, a-b.) Upper Carbonic. 

 Pedicle valve uniformly convex with beak scarcely projecting 



above the hinge line. Mesial fold and sinus absent. Surface 

 marked with fine radiating striae and with a few scattered spines. 

 Throughout the Upper Carbonic of North America. 



Fig. 297. a, b, Produclus cora ; c-e, P. costatus; f-g, P. 

 Geol. Surv. ) 



nspina. ( Ind. 



154. P. costatus (Sowerby ?) de Koninck. (Fig. 297, c-e) 



Upper Carbonic. 



Of medium size. Pedicle valve gibbous with a broad, shallow 



sinus producing a slight emargination at the anterior border. 



Beak prominent but only slightly projecting over the hinge mar- 



