128 INVERTEBRATE PALEONTOLOGY. [WHITE. 



Specific name given in honor of Prof. J. W. Powell, geologist in charge 

 of the Second Division United States Geological and Geographical Survey. 



Position and locality. Tertiary strata, probably late Eocene ; Bijou 

 Basin, forty miles east of Denver, Colorado. 



Genus PISIDIUM Pfeiffer. 



Pisidium saginatum (sp. nov.). Shell small, subcircular in marginal 

 outline ; anterior side slightly longer than the posterior ; valves inflated, 

 the convexity from beak to base being sometimes irregular in consequence 

 of one or more abrupt concentric flexures. 



Surface marked by ordinary lines of growth. 



Length, five millimeters ; height, five and a half millimeters ; thick- 

 ness, five and a half millimeters. 



Position and locality. Bitter Creek Group; Alrny coal mines, near 

 Evanston, Wyoming. 



Genus MESODESMA Deshayes. 



MesQdesma Bishopi (sp. nov.). Shell small, subovate or subtrihedral in 

 marginal outline, moderately gibbous ; umbonal ridges somewhat distinct, 

 the posterior pair more so than the others ; umbones prominent ; beaks 

 small ; both anterior and posterior lateral teeth well developed and trans- 

 versely striated, as in Corbicula; cartilage pit beneath the beak small; car- 

 dinal tooth in front of it rather small and V-shaped; pallial sinus deep; 

 muscular scars distinct ; right valve unknown. 



Surface nearly smooth, and marked by very fine lines of growth. 



Length, about one centimeter ; height, about six millimeters. 



This shell differs from typical forms of Mesodesma in its V-shaped car- 

 dinal tooth, transversely striated lateral teeth, and deep pallial sinus. 



Specific name given in honor of Prof. F. M. Bishop, of Salt Lake City, 

 Utah. 



Position and locality. Tertiary strata, probably late Eocene ; Bijou 

 Basin, forty miles east of Denver, Colorado. 



