126 INVERTEBRATE PALEONTOLOGY. [WIIITR. 



the middle; posterior extremity abruptly rounded; dorsal margin broadly 

 rounded obliquely downward to the posterior extremity ; the dorsum of each 

 valve elevated and its margin flexed inward and downward to the cardinal 

 ligament, so that the latter is hidden from sight by side view of the shell. 



Surface marked by the ordinary lines of growth and by numerous fine 

 radiating striae, which appear also in the substance of exfoliated portions of 

 the test 



Length, five centimeters; height from -base to umbones, thirty-seven 

 millimeters. 



This species is of the type of U. clavus Lamarck, which it much resem- 

 bles in general aspect. It is so different from any other known species of 

 Unio in the Tertiary rocks of America that it cannot be mistaken for any 

 of them. 



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



Unio bracliyopistlms (sp. nov.). Shell small or of medium size, some- 

 what gibbous, subcircular in marginal outline, the length and height being 

 about equal ; umbones broad, not prominent ; beaks depressed, situated 

 near the middle of the dorsum ; postero-dorsal portion broad, depressed so 

 that rounded timbonal ridges are formed, which extend to the postero-basal 

 extremity, and the hinge ligament is hidden from sight by side view of the 

 shell. 



Surface marked only by the ordinary lines and lamellations of growth. 



Length and height of the largest example discovered, each forty-four 

 millimeters. 



This species may be readily distinguished from all others by its sub- 

 circular, marginal outline and its extremely short and abruptly-rounded 

 posterior. The shortness of the posterior portion does not appear so con- 

 spicuously in the young shell as in the adult, because the additions by 

 growth are made more rapidly upon the basal and antero-basal borders 

 than elsewhere. 



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



Unio Slioslionensis* (sp. nov.). Shell of ordinary size, broadly subel- 



* The so-called tribal name is applied by the Indians themselves to their country or laud, riot to 

 the tribe. 



