DESCRIPTIONS OF NEW SPECIES. 125 



TERTIARY PERIOD. 

 Mollusca. 



Conchifera. 



Genus UNIO Retzius. 



Uniopetrinus (sp. nov.). Shell very large, transversely elongate, moder- 

 ately thick; test massive; basal and dorsal margins subparallel; the latter 

 broadly but slightly convex and the former nearly straight or faintly emargi- 

 nate about, or a little behind the middle; front abruptly rounded; postero- 

 dorsal and postero-basal margins, somewhat abruptly rounded to the pos- 

 terior margin, giving, in some cases, a subtruncate appearance to the posterior 

 end of the shell; beaks depressed, situated near the anterior end; umbones 

 broad; hinge massive, both cardinal and lateral teeth being very strong. 



Surface apparently marked in no other manner than by the ordinary 

 lines and imbrications of growth. The outer prismatic layer is well preserved, 

 and the umbones, like those of all the species of Unio I have examined, from 

 the Mesozoic and Cenozoic strata of that region, appear to have suffered, 

 while living, no erosion, such as is common in the case of the recent Unio- 

 nidae of the Mississippi and its tributaries. 



Length of the largest example in the collection, fifteen centimeters ; 

 height of the same, seven and a half centimeters. In the case of young 

 examples the length is proportionally greater. 



This species may be readily distinguished from all others at all likely 

 to be confounded with it by its great size, elongate form, and its subparallel 

 dorsal and ventral margins. 



Position and locality. Bitter Creek Group; Black Buttes, Wyoming. 



Unio proplieticus (sp. nov.). Shell small or of medium size, obliquely 

 subovate in marginal outline, moderately thick, the greatest thickness being 

 a little below the umbones; test rather thick; umbones prominent, directed 

 forward; beaks curved inward and forward, reaching as far as, or a little 

 farther than, the front of the shell; front broad, nearly perpendicular; front 

 margin slightly convex above, but abruptly rounded to the basal margin 

 below ; basal margin broadly rounded, or sometimes a little straightened at 



