66 GEOLOGICAL SURVEY OF THE TERRITORIES. 
versely somewhat flattened below the beaks; test of ordinary thickness; 
basal and dorsal margins subparallel, the latter being broadly convex, 
and the former more nearly straight, or sometimes a little emarginate; 
front margin regularly rounded from beneath the beaks to the basal 
margin ; posterior margin nearly straight or slightly convex, truncating 
the shell obliquely upward and backward, abruptly rounded to the basal 
margin and somewhat sharply rounded to the dorsal margin; beaks 
moderately strong, not elevated, but incurved and projecting beyond 
the front of the shell; the antero-dorsal and umbonal portions of each 
valve are sufiiciently elevated to hide the cardinal ligament when the 
shell is seen by side view, but the postero-dorsal portion of each valve 
slopes away directly from the dorsal margin. Two obtuse ridges or ele- 
vations radiate from the beak of each valve to the margin. One of these, 
the umbonal ridge proper, ends at the junction of the posterior and basal 
margins, and the other at the junction of the posterior and dorsal margins. 
Between the latter ridge and the dorsal margin the space is narrow; 
between the two ridges the space is wider and distinctly flattened, and 
forward of the umbonal ridgethereisan umbonal flattening, or very broad 
and very shallow unbonal sinus, which, extending from the umbo to the 
basal margin, causes the slight emargination there that has been referred 
to. Cardinal teeth moderately strong; lateral teeth well developed. 
Surface marked by the ordinary lines and undulations of growth, and 
faint radiating lines are also usually observable, especially where the 
prismatic layer has been exfoliated. 
Length from front to postero-dorsal prominence, 52 millimeters ; height 
from basal margin to dorsum, 35 millimeters; greatest thickness, both 
valves together, 31 millimeters. 
This species is related to U. propheticus White, as alreaily noticed in 
connection with the description of that species, but the differences are 
well characterized in the description of each, and they are also well 
shown by the figures on plate 22. The extreme anterior position of the 
beaks in these two species is a common characteristic among the fossil 
Uniones of Western North America, from the Jurassic to the Hocene 
Strata inclusive. This feature is not unknown among living North 
American forms of Unio, but it is proportionally less common among 
them than among the fossil forms. 
Position and locality—Upper strata of the Laramie Group, Black 
Buttes Station, Union Pacific Railroad, Wyoming, where itis associated 
with all the previously described species of this article, and also with a 
part of the following. 
UNIO ENDLICHI White. 
Plate 26, figs. 1 a and Bb. 
Unio endlicht White, 1877, Bull. U. S. Geol. Sur. Terr., vol. iii, p. 604. 
Shell large, obliquely elongate, subovate in marginal outline, moder- 
ately thick, very short in front of the beaks and elongate and rapidly 
narrowed behind them; test strong, and massive in old shells; basal 
margin having a slight general convexity, but it is straightened, and 
sometimes a little emarginate at or a little behind the mid-length, regu- 
larly and continuously rounded to the front, and abruptly rounded to 
the postero-basal margin; dorsal margin proper rather short, broadly 
rounded from front to rear; postero-dorsal margin forming a long, gently 
_ convex downward slope to the posterior margin, which latter margin is 
very narrowly rounded ; beaks much depressed, scarcely distinguishable 
