1915.] Cockerell, New Fossil Species of Unio. 123 
Unio eomargaron n. sp. 
Shell elliptical, with the ends broadly rounded, the lower margin (except toward 
the ends) straight, the upper nearly so; beaks (no sculpture visible) not far from 
anterior end, little prominent; valves only moderately convex, little flattened 
posteriorly; growth lines distinct, but no other sculpture; shell only moderately 
thick, in some specimens brilliantly pearly-iridescent; dentition broad but not heavy 
(left valve), resembling that of U. washakiensis Meek. Length 60 mm., depth 
about 32.5, width about 22. 
Eocene; head of Big Sand Coulee, in coarse friable greenish sandstone, 
in numbers (W. Stein, a 8, 1912). Clark Fork Beds. They are also 
labelled “Ralston.” °«~' 
This is certainly allied to U. washakiensis Meek, but is a lighter shell, 
with the umbones rather more posterior, and the posterodorsal region less 
elevated. I examined U. washakiensis Meek in the U.S. National Museum, 
Fig. 2. Unioeomargaronn.sp. A, Outline of shell; B, dentition of left valve. 
and noted as follows: “A large quantity of material, Wyoming and N. W. 
Colorado (near Raven Park and White River Valley). However the 
specimens from Washakie Station appears to me to be different from the 
rest; one of these (Washakie Sta., 18996) shows remnants of beak sculpture 
which was certainly plicate (undulate). The shell is very thick and deep, 
with heavy teeth; only the Washakie Sta. specimens show the hinge. All 
the other so-called washakiensis are longer, more parallel in outline, rather 
U. clinopisthus-like shells. 'They may be the same, but it seems doubtful. 
One young one (18677, eight miles below Green River Station) shows 
strongly plicate (undulate) beak sculpture. The Colorado specimens seem 
to be the same as this 18677, but they are all small, and apparently imma- 
ture. It is not quite clear that the Washakie Sta. specimens are different 
from U. haydeni.”’ 
With regard to U. haydeni Meek, I noted as follows at the U. 5S. National 
Museum: “8869 (Cotypes); near Fort Bridger, Wyo.; deep, flattish species, 
