468 



NORTH AMERICAN INDEX FOSSILS. 



287. G. corrugata Say. (Fig. 624.) Comanchic. 



Lower valve boat-shaped and thick. Growth lines coarse. 

 Dorsal sinus distinct. Beak usually slightly twisted toward dor- 

 sal margin. Upper valve somewhat triangular and thick. 



Fig. 622. Gryphcca tnexicana. a, side view of a small lower valve ; b, a larger 

 lower valve. (After Cragin. ) 



Washita of southwestern North America from Kansas to Mexico. 

 Especially common in the Preston of Texas. 



Three varieties of G. corrugata have been recognized, distin- 

 guished mainly by differences in size. 



Fig. 623. Gvyphaa marcoui. (After Hill.) 



287a. G. corrugata var. hilli Cragin. Comanchic. 



Small, resembling the young of G. corrugata. 



Very abundant in the Belvidere beds of Kansas. 

 287^. G. corrugata var. tucumcarii Marcou. Comanchic, 



Large. 



