PELECYPODA—PRIONODESMACEA. 5 1 5 



Differs from M. mytiloides in its greater proportional height and 

 broader and less extended anterior end. 



Hamilton : New York, Indiana, Falls of Ohio. 



416. M. concentrica Hall. (Figs. 692, d; 694, d.) Devonic. 

 Of medium size. Distinguished by its strong and regular striae. 



Differs also from M. mytiloides in its smaller size and more promi- 

 nent umbos ; from M. complanata in its more nearly straight base ; 

 from M. alta in its more elongate form. 



Hamilton : New York and Maryland-Indiana and Wisconsin. 



417. M. subalata Hall. (Fig. 692, b t ) Devonic. 

 Of medium size or smaller. Distinguished by its more nearly 



parallel cardinal and basal margins, angular umbonal ridge, and 

 the obsolescence of the striae on the umbonal region. 



Hamilton : New York, Pennsylvania ; also in Marcellus and 

 Ithaca of New York. 

 417^. M. subalata var. chemungensis Hall. Devonic. 



Differs from the species in being longer in proportion to the 

 height, cardinal line less oblique, its posterior extremity usually 



FlG. 693. Modiolodon oviformis. (Ohio, VII.) 



more rounded, umbonal slope more arcuate, and striae are filiform, 

 sharper and more regular. 



Characteristic of Portage of New York, Pennsylvania. 

 418. M. quadrula Hall. (Fig. 692, a.) Devonic. 



Small, quadrangular, with straight umbonal ridge. 



Chemung : New York. 



