ART. XKI.-PALEONTOLOGICAL PAPERS NO. 2. -DESCRIP- 

 TIONS OF NEW SPECIES OF UNIONES AND A NEW GENUS OF 

 FRESH-WATER GASTEROPODA FROM THE TERTIARY STRATA 

 OF WYOMING AND UTAH. 



By C. a. AYniTE, M. D. 



UNIONID.E. 



Unio proayitus (so. uoy.), — Shell of medium size, moderately ventri- 

 cose, irregularly oblong or subtetraliedral in marginal outline; front 

 moderately broad and flattened below the beaks ; test of ordinary thick- 

 ness; basal and dorsal margins subparallel, the latter being broadly 

 conYex, and the former more nearly straight or slightly emarginate; 

 front margin regularly rounded from beneath the beaks to the basal 

 margin ; posterior margin nearly straight or slightly convex, truncating 

 the shell obliquely upward and backward, shortly rounded to the basal 

 margin and more sharply rounded up to the dorsal margin ; beaks mod- 

 erately strong, not elevated, but incurved and projecting beyond the 

 front margin of the shell ; the anterodorsal and umbonal portions of 

 each valve are sufficiently derated to hide the cardinal ligament from 

 a side-view of the shell, but the postero-dorsal portion of each valve 

 slopes away from the dorsal margin. Two obtuse ridges or elevations 

 radiate from the beak of each valve to the margin. One of these, the 

 umbonal ridge proper, ends at the junction of the posterior and basal 

 margins, and the other at the junction of the posterior and dorsal mar- 

 gins. Between the latter and the dorsal margin, the space is narrow ; 

 between the two ridges, the surface is distinctly flattened, and forward 

 of the umbonal ridge there is an umbonal flattening or very shallow 

 umbonal sinus, which causes the slight emargiuation of the ventral 

 margin as well as the flattening of the outer side of the beaks. Cardi- 

 nal teeth moderately strong ; lateral teeth well developed. 



Surface marked by the ordinary lines and undulations of growth, and 

 faint radiating lines are also usually observable, especially where the 

 prismatic layer has been exfoliated. 



Length from front to postero-dorsal prominence 52 millimeters; 

 height from basal margin to dorsum 35 millimeters ; greatest thickness, 

 both valves together, 31 millimeters. 



This species is related to U . prophcticus White, but difl'ers from that 

 species in having an oblong instead of ovate outline; in its nearly hor- 



