208 TERTIARY PERIOD. 



somewhat compressed laterally; front margin abruptly and basal margin 

 broadly rounded ; posterior side sloping downward and backward to the 

 jjostero-basal margin, where it is narrowly rounded to meet the basal margin ; 

 beaks elevated, pointed, curving inward and forwai'd, and nearly meeting 

 each other across the hinge. The dorsal margin of each valve behind the 

 beak is bent more or less deeply and sharply inward and downward, fonn- 

 ing a prominent curved umbonal ridge, which extends from the beak to 

 the postero-basal border, and completely obscures the line of union of the 

 two valves along the postero-superior margin, as the shell is viewed laterally. 

 This last-named character is a marked feature of the shell, and gives the 

 appearance of unusual elevation to the umbonal ridges. 



Surface marked by distinct, concentric lines of growth. 



Length from front to postero-basal margin, forty-nine millimeters ; 

 height, from base to top of umbo, forty-two millimeters. 



Position and locality. — Strata probably of Eocene Tertiary age; plateau 

 near the west crossing of the north fork of Vii'gin River, at Pace's Canon, 

 and at the north fork of Vii-gin River, Utah. 



Genus SPH^RIUM Scopoli, 1777. 

 Sphaerium (?). 



Among the collections made from Tertiary strata west of Fairview, 

 Utah, are some specimens of SphcBrium that appear to belong to S. formosum 

 Meek and Hayden, the type-specimens of which species were obtained from 

 Tertiary strata near Fort Union, Nebraska. They are, however, too imper- 

 fect, and the specific characters too inconspicuous to allow their identity to 

 be recognized with certainty. 



