434 NON-MARINE FOSSIL MOLLUSCA. 



have been collected, by Professor Powell, from certain strata in the 

 Caiion of Desolation, ,nd others, by one of the parties under Lieuten- 

 ant Wheeler, from the coal-bearing strata at Wales, Utah, all of which 

 probably belong to the upper portion of the Laramie Group. The speci- 

 mens obtained at Wales were erroneously referred by me to U. vetustus 

 Meek,* which species U. tnendax resembles in certain particulars. 



In the Eocene fresh- water deposits, which immediately succeeded the 

 Laramie, several species of Unio have been found 5 and it is a note- 

 worthy fact that while there was great diversity of form and consider- 

 able diversity of surface ornamentation among the species of Unio that 

 existed in the Laramie period, all the species of that genus which have 

 yet been found in the purely fresh- water Eocene deposits have a trans- 

 versely oval form and plain surface; that is, there are among the latter 

 no short forms, like several of those of the Laramie Group and many 

 among living species, and none of them have the surface marked by 

 plications, rugae, or pustules; nor are they marked in any other manner 

 except by the ordinary concentric lines of growth. Most of these oval 

 shells also have the beaks placed at least a moderate distance from the 

 front, as is usual in the case of living species of oval form; but one of 

 them at least possesses a shortened front, like that of several of the 

 Uniones of the Laramie Group which have already been noticed. 



The species referred to is Unio clinopisthus White, which is described 

 and figured for the first time, as follows: Unio clinopisthus (sp. nov.), 

 Plate 28, Figs. 1 and 2. Shell transversely elongate, short in front of 

 the beaks, elongate and narrowing behind them to the posterior end; 

 basal margin having a gentle sinuosity, there being a slight einargina- 

 tion just behind the midlength ; front margin regularly rounded; dor- 

 sal margin proper rather short ; postero-dorsal margin forming a long, 

 convex, downward slope from the dorsal to the postero-basal margin, 

 which latter margin is narrowly rounded; beaks depressed and placed 

 near the front of the shell. A somewhat prominent, but not sharply 

 defined, umbonal ridge extends from the beak of each valve to the 

 postero-basal margin, giving a flattened space at the postero-dorsal por- 

 tion of each valve. Surface marked only by concentric lines of growth. 



Length, 63 millimeters; height, 30 millimeters; thickness, both valves 

 together, 23 millimeters. 



This species was collected by Dr. Hayden from the fresh- water Eocene 

 strata, near Washakie Station, in Southern Wyoming. 



Another species, quite a large one, to which I have given the name 

 of Unio Meekiij changing it from U. leai Meek, which name was pre- 

 occupied, was obtained by Dr. Hayden from the Bridger Group of South- 

 ern Wyoming. This form is yet known only by fragments and natural 

 casts of the interior; but it is known to have been a large, plain shell 



*Expl. and Sur. West of the 100th Merid., vol. iv, p. 206, pi. XXII, fig. 12, a, b, c, 

 andd. 



