372 REVIEWS 
The Jurassic assemblage forms the most important element of the 
collection. The two chief fossiliferous areas are: the one in the north- 
west corner of the Park, on the head waters of the Gardiner and Gal- 
latin Rivers; and the other on the slopes of Sheridan Peak and 
farther southwest of Snake River. Two zones, characterized more by 
lithological than faunal peculiarities, are to be recognized, but the fos- 
sils belong to a single fauna. 
The upper zone is marked by an arenaceous limestone yielding an 
abundance of Rhynconella gnathophora, R. myrina, Ostrea strigilecula, 
Camptonectes bellistriatus, and C. pertenuistriatus. The lower zone is 
characterized by calcareous clays and marls containing the majority of 
the above forms associated with Pleuromya subcompressa, Pholadomya 
Aingt, and Gryphea calceola var. nebrascensts. 
I found very similar zones in the Freeze-Out Hills of Wyoming, 
but they were characterized by slightly different assemblages of fossils. 
The upper zone consisted of clays and arenaceous limestones contain- 
ing Pentacrinus astertcus in abundance, and Asterias dubium, Camptonec- 
tes bellistriatus, C. (extenuatus) pertenuistratus, and Ostrea strigtlecula. 
In the lower zone occurred clays and marls with calcareous nodules 
yielding Astarte packardt, Pinna kingi, Pleuromya subcompressa, Phola- 
domya kingt, and other forms. Gelemnites densus and Pentacrinus astert- 
cus is common to both zones. 
As these zones are both extremely narrow, are composed largely of 
clastic material, and contain an assemblage of fossils in many instances 
common to both, I think the conclusion that but a single fauna is rep- 
resented is the correct one. This conclusion in regard to the Yellow- 
stone region Dr. Stanton extends to the entire Jurassic formation of 
the Rocky Mountain region, and concludes as follows: ‘The strati- 
graphic relations and the geographic distribution of the marine Juras- 
sic of the Rocky Mountain region are in favor of the idea that all of 
these deposits were made contemporaneously in a single sea.” 
A thin stratum of limestone in a position above the Jurassic beds 
and not far below the base of the Cretaceous section contains fresh 
water gastropods and Unios. ‘The formation which contains this lime- 
stone is referred with considerable doubt to the Dakota. It is thought 
that it may be the equivalent of the Kootenai or Como. A similar 
limestone stratum occupying approximately the same stratigraphic 
position is found in the Como of Wind River, of the Black Hills, and 
the writer found it also in the Freeze-Out Hills. In all these localities 
