64. GEOLOGICAL SURVEY OF THE. TERRITORIES. 
The elongate, subtrihedral outline, prominent, angular, and wrinkled 
umbonal ridges, and bread, flattened dorsum of this species, are features 
which separate it distinctly from all other known forms, whether tossil 
or recent, and, together with the last and the six or eight next described 
species, which are associated with it, shows an extent and diversity of 
differentiation among these earlier species of Unionidee that is hardly 
surpassed among living forms of that family even in North American 
fresh waters. 
Position and locality.—Upper part of the Laramie Group, Black Buttes 
Station, Union Pacific Railroad, Wyoming. 
UNIO BRACHYOPISTHUS White. 
Plate 22, figs. 2 a and b. : 
Unio brachyopisthus White, 1876, Powell’s Rep. Geol. Uinta Mts., p. 126. 
Shell small or of medium size, somewhat gibbous, subcireular in mar- 
ginal outline, the length and height being about equal; umbones broad, 
not prominent; beaks depressed, situated near the middie of the dor- 
sum; postero-dorsal portion of the shell broadly flattened, or so de- 
pressed that rounded or subangular umbonal ridges are formed, bound- 
ing the flattened or depressed posterior; cardinal margin short, and 
that portion of it behind the beaks is usually so depressed that the hinge 
ligament is hidden from sight by side view of the shell. Surface marked 
only by the ordinary concentric lines and laminations of growth. 
Length and height of the largest example discovered, each 44 millime- 
ters; but the example figured on plate 16 is much smaller. 
This species may be readily distinguished by its subcircular outline 
and its very short, abruptly-sloping, and rounded posterior. The pos- 
terior portion of the young shell of this species is not proportionally so 
short as that of the adult, because the additions by growth are made 
more rapidly upon the basal border than upon that of the postero-basal 
region; aS is commonly the case with shells of this genus. 
Position and locality—Upper strata of the Laramie Group, Black 
Buttes Station, Union Pacific Railroad, Wyoming, where it is associated 
with the preceding and most of the following-deseribed species: 
UNIO COUESI White. z 
Plate 27, fig. 1 a. 
Unio petrinus White (not Gould, 1855), 1876, Powell’s Rep. Geol. Uinta Mts., p.-125. 
Unio couesi White 1877, Bull. U. S. Geol. Sur. Terr., vol. iii, p. 605. 
Shell very large, transversely elongate, moderately thick; test mas- 
sive in fully adult shells; basal and dorsal margins subparallel, the latter 
broadly or slightly convex and the former more nearly straight or faintly 
emarginate a little behind the mid-length; front abruptly rounded; pos- 
tero-dorsal and postero-basal margins both somewhat abruptly rounded 
to the posterior margin, giving in most cases a subtruncated appearance 
to the posterior end of the shell; beaks depressed, scarcely distinguish- 
able as such, situated in advance of the mid-length of the shell; umbones 
broad, not prominent; hinge massive, both cardinal and lateral teeth 
being strong and well developed. Surface marked only by the usual 
concentric lines and imbrications of growth, which become coarse and 
prominent in old shells. The outer prismatic layer is well preserved on 
some of the examples, and the same has been detected upon all the other 
