296 



NORTH AMERICAN INDEX FOSSILS. 



Indiana, Illinois, Iowa, Missouri, Kansas, Arkansas, Nebraska, 

 Utah. 



LXXXIII. Eatonia Hall. 



Anterior margin deeply sinuate. Pedicle valve nearly flat with 

 a broad deep sinus and small, perforate beak ; muscle scars large 

 and deeply excavated. Brachial valve convex and often ventricose ; 

 cardinal process very long, resting upon a short median septum 

 and bifid at summit. Differs from Rliynclionella in the absence of 

 dental plates, in the presence of a cardinal process and in the 

 stronger muscle impressions of the pedicle valve. Devonic. 



279. E. medialis (Vanuxem). (Fig. 368.) Devonic. 



Hinge line nearly straight. Brachial valve regularly convex 



from beak to front and elevated into a broad median fold. Pedicle 



valve somewhat convex at umbo and thence concave to the front 



Fig. 36S. Eatonia medialis (Pal. N. V., III.). 



and depressed into a broad sinus. Surface marked by 12-16 broad 

 plications of which 4 are on the summit of the fold and 3 in the 

 bottom of the sinus. 



Helderbergian of Maine, New York. 



280. E. peculiaris (Conrad). (Fig. 369.) Devonic. 



Longitudinally ovate. Hinge line much shorter and more 

 rounded than in E. medialis ; the mesial fold and sinus are much 

 less pronounced and the margins of the valves are denticulate. 

 Surface marked by fine radiating striae. 



