PELECYPODA—PRIONODESMACEA, 



433 



192. M. fibristriata White and Whitfield. (Fig. 567, c.) 



Upper Devonic and Mississippic. 



Differs from M. chemungensis in its straighter form, more erect 

 beaks, and thread-like undulating radii. 



Chemung : Iowa. Mississippic : Michigan, Ohio, Iowa. 



Fig. 567. a, b, Mytilarca chemungensis, right valve showing striated ligament area 

 and left valve showing small cardinal teeth ; c, M. fibristriata, left valve ; d, <?, Pletho- 

 mytilus oviformis. (Pal. N. Y. ; V.) 



LIX. Plethomytilus Hall. 

 Equivalve, very inequilateral. Hinge line straight. An obscure 

 posterior ear present. Surface covered with concentric striae. 

 Ligament external, its area striated. Lateral teeth oblique ; no 

 cardinal teeth present. Pallial line simple. Devonic. 



