PELECYPODA—PRIONODESMACEA. 383 



Differs from G. alveata in its smaller size and in the presence of 

 concentric folds on the posterior slope. 



Hamilton : New York, Pennsylvania, Virginia. 



25. G. arcuata (Conrad). (Fig. 486, e-f.) Devonic. 

 Distinguished by its somewhat elongate form, rounded posterior 



extremity, flattened or constricted base, and by the extension of 

 the concentric folds across the whole length of the shell. 

 Hamilton of New York and the Appalachian region. 



26. G. circularis Hall. (Fig. 486, g.) Devonic. 

 Subcircular, with lightly impressed fold and furrows extending 



from beak to base. 



Hamilton and Chemung of New York. 



27. G. communis Hall. (Fig. 486, h.) Devonic. 



1 



FlG. 487. Grammysia alveata. (Pal. N. Y., V.) 



Elongate, with subangular umbonal slope, oblique sinus, and 

 strong concentric undulations. 



Chemung : New York, Pennsylvania. 



28. G. undata Hall. (Fig. 486, i.) Devonic. 

 Similar to G. communis, but has fewer concentric undulations, 



a less angular umbonal slope, broader posterior end, and a more 

 nearly vertical sinus. 



Chemung : New York, Pennsylvania. 



29. G. hannibalensis (Shumard). Mississippi. 

 Similar to G. communis but is higher proportionally, and has a 



more broadly rounded umbonal slope. 



Characteristic of the Waverly and equivalent formations from 

 Ohio to Missouri ; also in the Mississipic of Nevada. 



