604 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



izontal instead of oblique dorsal margin f in possessing the radiating 

 ridges before described ; and in having the umbones and dorsal portions 

 of its valves less elevated. 



Position and locality. — Wahsatch gronp ; — Black Buttes Station, Wyo- 

 ming Territory, where it is associated with the two following species. 



Unio holmesianus (sp. nov.). — Shell small, short, compact, moder- 

 ately gibbous, subtrihedral in marginal outline, of ordinary length anteri- 

 orly, very short posteriorly, the postero-dorsal portion forming only a 

 small, narrow prominence upon the broad, abruptly truncated posterior; 

 valves in front of the umbonal sinus regularly convex ; umbonal sinus 

 well defined but somewhat narrow, its posterior side a little higher than 

 the anterior, terminating at the base of the shell a little behind the 

 middle; umbonal ridge prominent, forming a roughly rounded angle 

 between the side of the valve and its posterior end ; hinge-line very 

 short ; antero-dorsal margin sloping gently downward ; front and antero- 

 basal margins regularly and continuously rounded to the termination 

 of the umbonal sinus, where the base is a little emarginate; postero- 

 basal margin sharply rounded up to the almost perpendicular posterior 

 margin ; beaks small, somewhat prominent, incurved. The surface of 

 a moderately narrow space along the front and basal margins as far 

 back as the umbonal sinus is marked only by the ordinary lines of 

 growth ; all the remainder of the surface in front of the umbonal sinus 

 is marked by somewhat close-set rhombic papillte, arranged in more or 

 less distinct oblique lines. All the surface behind the umbonal sinus 

 marked by small, irregular, somewhat corrugated ridges, which are 

 strongest upon the umbonal ridge, from the median line of which they 

 diverge downward toward both the umbonal sinus and the posterior 

 margin. 



Length 23 millimeters; height 21 millimeters; thickness 14 milli- 

 meters. 



This species presents a greater degree of differentiation from what 

 may be regarded as the typical form of the genus than any other known 

 fossil species, and fully as great as that of any species now living. 



The specific name is given in honor of Mr. W. H. Holmes, of the 

 United States Geological Survey of the Territories. 



Position andlocality. — Wahsatch group ; — Black Buttes Station, Wyo- 

 ming Territory, where it is associated with the foregoing species. 



Unio endlichi (sp. nov.). — Shell large, obliquely elongate-ovate in 

 marginal outline, moderately thick, very short in front of the beaks and 

 elongated and narrowed behind them; test massive; basal margin having 

 a slight general convexity, but it is sometimes straightened, or even a 

 little emarginate behind the middle, regularly and continuously rounded 

 to the front, and abruptly rounded to the postero-dorsal margin ; dorsal 

 margin broadly rounded from front to rear, forming a broadly convex 

 slope down to the postero-basal margin ; beaks much depressed, and the 

 whole umbonal region only slightlj^ elevated ; hinge well developed ; 

 cardinal teeth strong ; lateral teeth large and very long. 



