70 GEOLOGICAL SURVEY OF THE TERRITORIES. 
UNIO PRIMAVUS White. - 
Plate 29, figs. 3 a and b. 
Unio primevus White, 1877, Bull. U. 8S. Geol. Sur. Terr., vol. iii,*p. 599. 
Shell of medium size, broadly subovate in marginal outline when 
adult, but proportionally narrower when young; valves moderately 
convex, each having a faint umbonal sinus or radiating flattened space, 
which ends at the basal margin a little behind the mid-length of the 
shell; this sinus or flattened space is bordered posteriorly by a broad 
undefined, umbonal ridge, or slight radiating prominence which ends at 
the postero-basal border; beaks situated nearly equidistant from the 
anterior and posterior ends, or a little nearer to the anterior, prominent 
by reason of the sloping away from it of both the antero and postero 
dorsal borders as well as the sides; from the beaks to the postero-basal 
portion of the shell the margin is broadly convex; postero-basal margin 
abruptly rounded to the base, the latter margin being gently convex, or 
sometimes a little straightened where it is met by the umbonal sinus or 
flattened space; front regularly rounded; both cardinal and lateral teeth 
well developed; the cardinal tooth of the left valve passing into a pit 
in the right valve which is situated directly under the beak. The pos- 
terior end of the lateral portion of the hinge has a peculiar modification 
of the usual method of articulation in Unio, as shown by fig. 3), plate 29. 
It also ends by a thickening of the substance of the hinge and a round- 
ing of its end, instead of having the lateral teeth ending sharply as is 
usual in shells of the genus Unio. Surface marked by the ordinary 
lines and undulations of growth, and the postero-dorsal portion is also 
marked by irregular raised linear ridges that have a somewhat cor- 
rugated appearance, their direction being from the beak towards the 
posterior and postero-basal borders. Besides these markings exfoliated 
portions of the test show fine radiating lines. 
This shell may be readily distinguished by its broadly subovate out- 
line and the peculiar irregular raised lines on the postero-dorsal surface. 
The peculiar character of the end of the lateral portion of the hinge 
which is seen in this shell is also to be observed in U. stewardi White, 
from the Jurassic strata of Northern Utah, but the hinges of the fossil 
Uniones that have hitherto been found in the Mesozoic and Cenozoic 
rocks of Western North America do not otherwise show any differences 
from those which prevail among the living Uniones of North American 
rivers. 
Length of an adult example, 65 millimeters; height of the same from 
base to beaks, 49 millimeters. 
Position and locality—Ferruginous sandstones at the summit of the 
Judith River series of the Laramie Group; south of Cow Island, Upper 
Missouri River, Montana, where it was collected by Prof. H. D. Cope in 
1876. 
UNIO PRISCUS Meek & Hayden. 
This species was discovered by Dr. Hayden in strata of the Judith 
River series of the Laramie Group, in the valley of Yellowstone River, 
and it has hitherto been discovered nowhere else. It is described and 
figured in vol. ix, U. 8. Geol. Sur. Terr. (4to ser.), p. 516, plate 43, 
figs. 3.a, b, ¢, and d. 
