WHItE.] LARAMIE FOSSILS. 67 
as such, and the whole umbonal region only slightly prominent; hinge 
well developed; cardinal teeth strong; lateral teeth large and very 
long. Surface marked by the ordinary concentric lines and imbrica- 
tions of growth, which are coarse and rough on old shells. 
Length of the largest example yet discovered, 130 millimeters; great- 
est height of the same, 70 millimeters. These proportions, and conse- 
quently the marginal outline, varied somewhat with their growth, young 
examples being longer in proportion to their height. The fragments in 
the collection also show that the outline of adult forms was subject to 
considerable variation in proportional length. 
This species resembles U. couesi, with which it is associated, in its 
large size, massive test, and simple surface ; but it differs materially in 
outline, being much narrower posteriorly, and having its beak much 
nearer to the front of the shell than it is in that species. It bears some 
resemblance also to some of the varietal forms of U. danw, Meek & Hay- 
den, but it is constantly a more massive shell than that species; is 
always proportionally wider or higher at its highest part, which is near 
the front; and it also has its beaks placed farther forward, the front of 
U. endlich projecting only very little beyond the beaks. 
Position and locality—Upper portion of the Laramie Group, Black 
Buttes Station, Union Pacific Railroad, Wyoming, where it is associated 
with the greater part of the species of Unio described in this article, as 
well as several species of Gasteropods. 
UNIO HOLMESIANUS White. 
Plate 22, figs. 4 a, b, c, d, and e. 
Unio holmesianus White, 1877, Bull. U. 8. Geol. Sur. Terr., vol. iii, p. 604. 
Shell small, or of medium size, compact, moderately gibbous, sub- 
trihedral in marginal outline, of ordinary length anteriorly for a short 
Shell, but very short posteriorly; the postero-dorsal portion forming only 
a small narrow prominence upon the broad, abruptly truncated posterior ; 
valves, in front of the umbonal sinus, regularly convex; umbonal sinus 
well defined but somewhat narrow, its posterior side a little higher or 
more distinetly defined than the anterior side, the sinus terminating at 
the base of the shell a little behind the middle; umbonal ridge promi- 
nent, forming a roughly rounded angle between the side of the valve 
and the posterior or postero-dorsal portion; cardinal margin very short; 
antero-dorsal margin sloping gently downward to the front of the shell, 
front and antero basal margins forming a regular and continuous curve, 
which extends to that portion of the basal margin where the umbonal 
sinus ends, where the base is a little emarginate; postero-basal margin 
a little roughened and abruptly rounded to the almost perpendicular 
or slightly oblique posterior margin; umbones somewhat prominent; 
beaks small, incurved; cardinal ligament moderately large. Until the 
shell had reached about one-third its full adult diameter the whole sur- 
face was marked by somewhat close-set rhombic papilla, which are ar- 
ranged in more or less distinct oblique lines, and which remain as the 
ornamentation of the upper and umbonal portions of the adult shell. 
All the surface below the portion thus marked by the papille and in 
front of the umbonal sinus is plain, or marked only by the ordinary dis- 
tinct lines of growth. All the surface behind the umbonal sinus is 
marked by small, irregular, somewhat corrugated ridges, which are 
stronger upon the umbonal ridge than elsewhere, and from the median 
line of which they sometimes diverge downwards, and towards both the 
