PELECYPODA—PRIONODESMACEA. 381 



by an oblique sinus or depression and adjacent fold, which may 

 be duplicate and which alternate in the two valves. Surface marked 

 with concentric growth lines and commonly with concentric undula- 

 tions. Hinge plate short and strong, with one or two cardinal folds 

 in the typical forms. Ligament external. Pallial line simple. 

 Siluric-Carbonic. 



18. G. ovata Hall. (Fig. 485, a.) Devonic. 

 Length about one third greater than height. Surface marked 



with subangular, concentric undulations. Sinus shallow and un- 

 accompanied by folds. 

 Onondaga : Ohio. 



19. G. bisulcata (Conrad.) (Fig. 485, b.) Devonic. 

 Distinguished by the strong fold with a furrow on each side, ex- 

 tending from beak to base and by the strong and regular concen- 

 tric ridges. 



Hamilton : New York, Pennsylvania. 



20. G. globosa Hall. (Fig. 485, c-d.) Devonic. 

 Distinguished by its comparatively small size and by its globu- 

 lar form. No fold or sinus. 



Hamilton : New York. 



21. G. nodocostata Hall. (Fig. 486, a-b.) Devonic. 

 Outline oblique ; hinge line short and straight. Surface marked 



with numerous concentric folds and on the umbo with radiating 

 nodose ridges. 



Hamilton : New York. 



22. G. obsoleta Hall. (Fig. 486, c.) Devonic. 

 Large, broadly ovate or elliptical from pressure, constricted at 



middle of base. Hinge line arcuate and more than half the length 

 of the shell. Sinus and ridge present anteriorly. Differs from G. 

 circularis in having the anterior fold and sinus and in not being 

 flattened. 



Hamilton : New York. 



23. G. alveata (Conrad). (Fig. 487.) Devonic. 

 Hinge line short, almost winged posteriorly. Valves convex 



below and ventricose above. Surface marked with fine concentric 

 striae and, anterior to the umbonal slope, by concentric folds which 

 are absent on the posterior slope. 



