164 PALJ^.ONTOLOGY. 



MOLLUSCA. 

 LAMELLIBRANCHIATA. 



UNIONIDJ]. 



Genus UNIO, Retzius. 



Unio vetustus, Meek. 



Plate 16, figs. 5, 5 fl, 5 b, and 5 c. 



Unio vetustus, Meek (1860), Proceed. Acad. Nat. Sci. Pbilad., VIII, 117 ; and (187G) in 

 Col. Simpson's Keport Expl. across the Great Basin of Utah, 359, pi. v, figs. 

 12 a, &.— White (1875), Eep. on Wheeler's collections 200, pi. xxi, figs. 12 a—d. 



Compare U. priscus, Meek and Hayden (185G), Proceed. Acad. Sci. Philad., 117.— Meek 

 (1876), Pal. Ui)p. Mo., 516, pi. 43, figs. 8 a, h, c. 



Shell of about medium size, transversely-subovate or subelliptic, the 

 widest pai-t being anteriorly, though young examples are narrower, with 

 dorsal and ventral margins more nearly parallel, rather thin and moder- 

 ately convex, about twice as long as high; base forming a long semi-elliptic 

 or semi-ovate curve in adult shells, but usually straighter in the young; 

 posterior margin rather narrowly rounded below in large specimens, and 

 obliquely truncated in small examples; dorsal margin nearly straight, except- 

 ing in large shells, where it is more arched; anterior margin short and 

 rounded; beaks depressed, not eroded, placed near the anterior, very neatly 

 ornamented with small, perfectly regular, concentric ridges and fuiTows, 

 that generally end abruptly behind at a small, oblique, linear, posterior 

 umbonal ridge extending backward and downward, while midway between 

 this and the cardinal margin there is a second similar ridge; other portions 

 of the surface merely marked with lines of growth, which sometimes assume 

 a subimbricating appearance near the free margins. Hinge of moderate 

 length, with two cardinal teeth in the left valve, the posterior one being 

 larger than the other; lateral teeth long and neariy straight (cardinal teeth 

 of right valve unknown) ; scars of anterior muscles deep and in-egularly 

 pitted. 



Length of a large specimen, about 4 inches; height, 2.23 inches; con- 

 vexity, about 1.10 inches. 



