194 STANTON—MOLLUSCAN FAUNULE. [April 8, 
zon of the Bear River formation—possibly contemporaneous with a 
part of it—and that it is certainly not older than Lower Cretaceous, 
and more probably should be assigned to about the base of the 
Upper Cretaceous. 
The facts thus briefly related should call renewed attention to the 
important and apparently complex history recorded in the non- 
marine formations of the late Jurassic and early Cretaceous of the 
Northwest—a history that is as yet far from being fully understood, 
although it is evident that the deposits contain the record of a 
great many facts that await the detailed investigation of the region 
for their interpretation. 
DESCRIPTION OF SPECIES. 
Unio FARRIn. sp. Pl. IV, Figs. 1, 2. 
Shell small, short and relatively convex; beaks somewhat prom- 
inent and inflated, situated about one-third the length of the shell 
from the anterior end, not sculptured nor eroded ; dorsal margin 
nearly straight ; ventral margin moderately convex; anterior end 
regularly rounded ; posterior end obliquely subtruncate ; umbonal 
ridges rather prominent, rounded, extending to the postero-ventral 
angle ; surface marked only by moderately prominent, irregularly 
arranged lines of growth. 
The type specimen measures 37 mm. in length, 22 mm. in 
height, and 16 mm. in convexity of both valves. The largest 
specimen of the eight in the collection is 45 mm. inlength. Three 
of the specimens are relatively somewhat more compressed and 
longer and have the posterior end more obliquely truncated. 
These differences are believed to be sexual rather than specific and 
are not greater differences than are seen in some living species of 
Unio. One of these compressed specimens (represented by Fig. 
2) measures 40 mm. in length, 22 mm. in height and 13 mm. in 
convexity. 
This species is suggestive of the parvus group of living Unios, 
but is not sufficiently closely related to any fossil species from our 
Western formation to require detailed comparison. 
Locality.—Wettacombe’s ranche near Musselshell River, in the 
vicinity of Harlowton, Montana. 
Horizon.—Upper part of Lower Cretaceous or base of Upper 
Cretaceous. 
