PELECYPODA—PRIONODESMACEA. 



377 



6. S. aeolus Hall and Whitfield. (Fig. 479.) Mississippic. 

 Two or three very obscure longitudinal ridges between the 



slightly convex umbonal slopes and the cardinal margin. 

 Ohio (Cuyahoga shale), Nevada. 



V. Prothyris Meek. 

 Thin-shelled, elongate, with very anterior beaks, and often nearly 

 parallel cardinal and basal margins. Lower anterior end gene- 

 rally notched. Surface covered with fine concentric striae. Hinge 

 without teeth. Ligament external. Pallial line simple. Devonic 

 and Carbonic. 



7. P. lanceolata Hall. (Fig. 477, e.) Devonic. 

 Length three times the height ; oblique, and pointed posteriorly. 



Anterior end subtruncate, only slightly constricted, and lacking 

 any conspicuous notch. 



Hamilton : New York, Pennsylvania. Portage : Pennsylvania. 



8. P. elegans Meek. Carbonic. 

 Anterior notch well defined. Surface marked with an obscure 



fold and furrow parallel to the hinge line. Striae nearly obsolete 

 On upper portion of shell. Length nearly three and one half times 

 height. Type of genus. 



Ohio and Michigan to Nebraska. 



VI. Orthonota Conrad. 

 Extremely elongate, with very anterior beaks. Cardinal and 



Fig. 480. Orthonota undulata. (Pal. N. Y., V.) 



basal margins straight and parallel. Umbonal slope defined by 

 one or more distinct folds which extend from the beak to the pos- 



