80 GEOLOGICAL SURVEY OF THE TERRITORIES. 
ciently distinct from the typical forms of Corbicula to warrant its sepa- 
ration as proposed by Meek, but there are known forms that seem to 
connect the two types by direct gradation. C. cleburni, C. Occidentalis, 
and C. Cytheriformis seem to be such connecting forms, but perhaps © 
they might with propriety be referred to Veloritina. 
Genus CORBULA Bruguiere. 
CORBULA SUBTRIGONALIS Meek & Hayden. 
CORBULA PERUNDATA Meek & Hayden. 
CORBULA CRASSATELLIFORMIS Meek. 
CORBULA TROPIDOPHORA MEEK. 
In the annual report of this survey for 1877, p. 170, I placed all four 
of the above-named forms under one species, C. subtrigonalis, and I have 
since seen no cause to change my views upon that point, although it is 
true that selected examples of each form present differences that would 
be necessarily regarded as specific if no intermediate forms were known 
to connect them too nearly for specific separation. It is also true that 
at different localities this species presents recognizable varietal differ- 
ences, but these are doubtless due to different environing conditions 
while the mollusks lived. 2 
Dr. Hayden obtained the type specimens of the two first-named forms 
from the Judith River series of the Laramie Group, in the Upper Mis- 
souri River region. They are deseribed by Meek in vol. ix, U. 8S. Geol. 
Sur. Ter. (4to ser.), pages 529 and 530, respectively, and figured on 
plate 40 of the same volume. 
Mr. Meek originally described both C. crassatelliformis and C. tropi- 
dophora from strata of the Bitter Creek series of the Laramie Group, the 
latter holding a position a few hundred feet lower in the series than the 
former. The latter is described in the annual report of this survey for 
1872, p. 514, and the former (under the name Corbicula? crassatelliformis) 
in the report for 1870, p. 315. Both these forms are figured on plate 25, 
accompanying this article, together with others from Crow Creek, Col- 
orado, east of the Rocky Mountains. 
CORBULA MACTRIFORMIS Meek & Hayden. 
This species seems to be distinct from either of the four preceding 
forms, and yet it may perhaps prove to be only a variety. It has been 
found only inthe Laramie strata near Fort Clark, in the Upper Missouri 
River region, where it was discovered by Dr. Hayden. It is described 
and figured by Meek in vol. ix, U. S. Geol. Sur. Terr. (4to ser.), p. 528, 
plate 43, figs. 7 a-f. 
CORBULA UNDIFERA Meek. 
Plate 29 figs 4 a, b, ¢, d, e, and f. 
Corbula undifera Meek, 1873, An. Rep. U. 8. Geol Sur. Terr. for 1872, p. 513. 
This well-marked species has proved to be a characteristic fossil of 
the Laramie Group west of the Rocky Mountains. The following is Mr. 
Meek’s description : 
“Shell of moderate size, trigonal subovate, rather convex, the in- 
equality of the valves not being very strongly marked, although always 
