STRATIGRAPHIC POSITION OF JUDITH RIVER FORMATION 741 
THE INVERTEBRATE EVIDENCE 
According to Dr. T. W. Stanton:™ ‘‘As the Judith River is 
essentially a non-marine formation, strictly speaking its fauna 
should not be made to include the marine species’? which in the 
one occurrence noted by him, he supposes to have been “‘ brought 
into the Judith River area by a local temporary invasion of pure 
marine waters.” ‘There is also in the formation ‘‘a brackish-water 
fauna of wide geographic distribution confined to thin beds in its 
upper and lower portions of the formation.” 
It is apparently the consensus of opinion among invertebrate 
paleontologists that fresh-water faunas per se are of little or no 
value in the accurate determination of the age of beds in which 
they occur. Fresh-water beds are found at a number of horizons 
between the Devonian and the present time. In the Devonian, 
shells resembling the modern Unio have been found and Unios of 
similar types have also been collected from the Triassic and Juras- 
sic. Writing of the fresh-water beds at the top of the Jurassic 
Dr. Stanton? says: 
Its invertebrate fauna consists of several species of Unio, Vivipara, Planor- 
bis, etc., all of modern fresh-water types, which do not assist in discriminating 
between Jurassic and Cretaceous. Unios have been found in several horizons 
in the Cretaceous, and when we get as high as the Ceratops beds (Lance for- 
mation) many, if not all of the specific types found there, may be found also 
among living species. 
Whitfield’ describing the Unios from the Hell Creek region of 
Montana says of fourteen species that they are “‘so nearly like the 
living species that it would do but little violence to specific features 
to say they were the same.”’ 
Writing of the non-marine faunas found in the Ceratops beds 
of Converse County, Wyo., Stanton says:4 
It must be admitted that in themselves, without any reference to strati- 
graphic occurrence or local geologic history, these fossils could not be depended 
t Bull. U.S. Geol. Surv., No. 257, pp. 119 f. 
2 Jour. Geol., XVII (1909), 414. 
3 Bull. Amer. Mus. Nat. Hist., XXIII (1907), 624. 
4“The Age and Stratigraphic Relations of the ‘Ceratops Beds’ of Wyoming 
and Montana,” Proc. Wash. Acad. Sci., XI (1909), 288. 
