PELECYPODA—ANOMALODESMACEA. 



527 



CXXIII. Endodesma Ulrich. 

 Shell very thin, equivalve, convex, elongate. Hinge and basal 

 margins subparallel. Mesial depression deep. Umbones com- 

 pressed, elevated. Hinge thin, apparently without teeth. A flat- 

 tened area (lunule) present in front of beaks. Surface covered 

 with concentric lines. Muscle scars very faint. Ordovicic. 



455. E. orthonotum Meek and Worthen. (Fig. 715, b, c.) 



Ordovicic. 

 Umbonal slope subangular dorsally. 

 Stones River limestone of Illinois and Minnesota. 



456. E. gesneri Billings. (Fig. 715, a.) Ordovicic. 



Fig. 715. a, Endodesma gesneri, right valve; b, c, E. orthonotum; d, e, E. 

 cuneatum, all internal molds. (Minn. Surv.) 



Differs from E. orthonotum in its subangular anterior extremity, 

 its more rounded upper posterior edge, and its comparatively 

 shorter length. 



Black River and Trenton of Ontario. 

 457. E. cuneatum Ulrich. (Fig. 715, d } e.) Ordovicic. 



Surface of internal mold bearing a few obscure concentric folds. 

 Type of genus. 



Middle Galena of Minnesota. 



CXXIV. Pholadella Hall. 

 Transversely elongate, with truncate posterior end and rounded 

 or obliquely truncate anterior end. Beaks anterior to middle and 

 prominent. Umbonal slope prominent. Surface marked with fine 



