206 TERTIARY PERIOD. 



CHAPTER X. 



CENOZOIC AGE. 



TERTIARY PERIOD. 



MOLLUSCA. 

 Class CONCHIFERA. 



Order DIMYARIA. 

 Family UNIONID^. 



Genus UNIO Eetzius, 1788. 



Unio vetustus Meek. 



Plate XXII, fig. 12 a, b, c, and d. 



TTnio vetustus Meek, 1860, Proc. Acad. Nat. Sci. Phila., 312. 



Uniopriseus Meek, 1870, Geol. Surv. Wyoming & contiguous Territories, 298. 



Shell somewhat elongate-ovate, some examples a little compressed, but 

 Old shells usually somewhat gibbous ; test moderately thick ; beaks placed 

 near the anterior extremity ; anterior margin regularly rounded down to 

 the basal margin, which is only slightly convex or a little straightened; 

 dorsal margin nearly straight or a little convex, and nearly parallel with the 

 base ; posterior margin obliquely sloping downward and backward from the 

 dorsal margin, and abruptly rounded below to meet the basal margin. 



Surface of young shells marked by numerous small, undulating con- 

 centric wrinkles, which cease to form as the shell increases in size, and are 

 then seen only upon the beaks ; surface of adult shells marked by the 

 ordinary lines of growth ; umbonal ridge rounded, not prominent ; two 

 somewhat sharply raised lines radiate from just behind the beak obliquely 

 downward and backward upon the space between the umbonal ridge and 

 the dorsal margin. 



