Ware. ] LARAMIE FOSSILS. 65 
species of Unio that have been found associated with it. Also, like all 
the species of Unio from the Mesozoic and Cenozoic strata of Western 
North America which I have examined, the umbones, even in the case 
of old shells, appear to have suffered no erosion, such as is common in 
the case of living Uniones of North American rivers. 
Length of the largest example discovered, 150 millimeters; height 
of the same, 75 millimeters. The length of young shells, compared 
with their height, is proportionally greater. 
This species was originally named U. petrinus (loc. cit.), but the name 
was subsequently changed to U. couesi, because the former name had 
been preoccupied by Gould for a living species of Unio. 
It may be distinguished from all other species that are in any way 
likely to be confounded with it, by its very great size, elongate form, and 
subparallel dorsal and basal margins. 
Position and locality—Upper portion of the Laramie Group, Black 
Buttes Station, Union Pacific Railroad, Wyoming, where it is associated 
with the majority of the species of Unio described in this article; besides 
several species of Gasteropods. 
UNIO PROPHETICUS White. 
Plate 22, fig. 5 a. 
Unio propheticus White, 1876, Powell’s Rep. Geol. Uinta Mts., p. 125. 
Shell not above medium size, obliquely subovate in marginal outline, 
moderately thick, the greatest thickness being a little below the um- 
bones; test moderately thick; umbones prominent; beaks reaching to 
or a little beyond the front of the shell, curved inward and a little for- 
ward; front nearly perpendicular; front margin slightly concave above, 
but abruptly rounded to the basal margin below; basal margin straight- 
ened or sometimes a little emarginate; posterior extremity abruptly 
rounded; dorsal margin broadly rounded, and forming, with the postero- 
dorsal margin, a continuous curve from the beaks to the posterior ex- 
tremity ; the portion of each valve which bears the hinge is so inflexed 
as to produce a dorsal concavity that hides from sight the hinge ligament 
when the shell is seen by side view. 
Surface marked by the ordinary lines of growth, and by numerous fine 
radiating striz, which appear more distinctly in the substance of partly 
exfoliated portions of the test. 
Length, 50 millimeters; height from base to umbones, 37 millimeters. 
This fossil species is of the type of the living VU. clavus Lamarck, which 
it much resembles in general aspect. It also resmebles, in some of its 
features, the next described species, U. proavitus, with which it is asso- 
ciated ; but it is so different from any other knowh fossil Unio, from 
American strata that it cannot be mistaken for any of them. 
Position and locality—Laramie strata, near the top of the group, Black 
Buttes Station, Union Pacific Railroad, Wyoming, where it is associated 
with the preceding, and also with the majority of the species of Unio 
which are described in this article. 
UNIO PROAVITUS White. 
Plate 22, figs. 3a, b, c, and d. 
Unio proavitus White, 1877, U. 8. Geol. Sur. Terr., vol. iii, p. 603. 
Shell of medium size, moderately ventricose, irregularly oblong or 
subtetrahedral in marginal outline; front moderately broad and trans- 
5H 
