PELECYPODA—PRIONODESMACEA. 



513 



412. M. dubia Hall. Siluric. 



Differs from M. orthonota in that the umbos are nearly at the 

 anterior extremity and the concentric striae are at times replaced 

 by stronger wrinkles. 



Manlius : New York. 



Fig. 689. Modiolopsis orthonota. (Pal. N. Y., II.) 

 FlG. 690. Modiolopsis primigenia. (Pal. N. Y., II.) 



CIX. Modiomorpha Hall. 

 Subovate, widest posteriorly, crossed obliquely from beak to 

 base by a more or less distinctly defined depression, constricting 



Fig. 691. 



a, Modiomorpha complanata, internal mold of left valve; i>, M. mytiloides ; 

 bothXM- (Pal. N. Y., V.) 



the basal margin. Beaks small, compressed. Surface marked by 

 rugose or undulating concentric striae. Hinge with strong wedge- 

 shaped tooth in left valve and corresponding cavity in right. No 



