295 
The base of the Hocene is usually regarded as containing a small per 
cent of the marine mollusks yet living; the beginning of the Miocene, 
about 17 per cent of yet existing species; and the beginning of the Pliocene 
about 36 per cent. If now plants have changed in species during the lapse 
of geological time with about the rapidity that marine mollusks have 
changed, the Fort Union beds ought to be arranged in the Lower Miocene. 
This would harmonize quite well with the idea that the Green River beds 
belong to the Oligocene. 
9. RELATIONSHIP OF LANCE CREEK FAUNA TO THAT OF THE JUDITH RIVER 
EPocuH. 
Having demonstrated, as I think I have, that there was, between the 
time of the deposition of the Lance Creek beds and those known as Puerco 
and Fort Union, a nearly complete change in the fauna and a considerable 
change in the flora, I will endeavor to show that the fauna of the former 
beds is closely related to that of the Judith River, a formation now recog- 
nized as being well down in the Upper Cretaceous and separated from the 
lowermost Laramie by about 1,000 feet of marine Cretaceous strata 
(Stanton, Wash. Acad. Sci., xi, p. 256). ‘This close relationship of the two 
faunas has been recognized, it may be truthfully said, by all paleontolo- 
gists who have given attention to the subject. For a long time it misled 
geologists and paleontologists into the conclusion that all the deposits in 
question belonged to a single epoch. Mr. J. B. Hatcher, who had collected 
extensively both in the Judith River region and in the Lance Creek beds, 
and who had studied closely the vertebrates of both regions, writes (Bull. 
U. S. Geol. Surv., 257, p.-101) : 
When considered in its entirety, the vertebrate fauna of these beds 
[Judith River] is remarkably similar to, though distinctly more primitive 
than, that of the Laramie [Lance Creek beds]. Almost or quite all of the 
types of vertebrates are present, though, as a rule, they are represented 
by smaller and more primitive forms. 
Doctor T. W. Stanton, palecntologist of the U. S. Geological Survey, 
who examined in company with Professor Hatcher the Judith River basin, 
and who has given especial attention to the invertebrate fauna, records in 
the same bulletin (p. 121) his opinicn: 
When full collections are compared it will usually be easy to distin- 
guish between Judith River and Laramie from the brackish-water fossils 
alone, but if the collections are meager and fragmentary it may not be 
