LATER MESOZOIC INVERTEBRATE FAUNAS AII 
area in British America. ‘The fauna of this Rocky Mountain Jurassic 
sea is characterized by Cardioceras cordijorme, Cadoceras, Belem- 
nites densus, and a rather varied though not large fauna consisting 
mostly of bivalves. It has long been recognized by Neumayr and 
others to be of boreal type and hence as indicating a connection either 
direct or indirect with the Arctic region. The fauna shows some 
local variations, usually associated with variations in the character 
of the sediments; but it appears to be essentially a unit throughout 
the entire area. It is believed that the deposits were all formed in 
one basin and within a comparatively brief period. ‘Their maximum 
thickness is usually only a few hundred feet. 
As there are no other marine Jurassic formations in the region and 
the section is known to be incomplete it is necessary to go to other 
areas where similar faunas occur to determine the exact position of 
this one in the general column. In the Upper Jurassic the following 
stages are recognized by De Lapparent who gives them universal 
application: 
Purbeckian 
Bononian 
Kimmeridgian 
Sequanian 
Oxfordian 
Callovian 
Portlandian ) 
The Jurassic of the Rocky Mountain region, as far as can be deter- 
mined from the fauna, represents the Oxfordian and perhaps the 
Callovian in whole or in part. That is, it is the lower part of 
the Upper Jurassic. In a large part of its area it rests on the Car- 
boniferous, and the youngest marine fauna found beneath it any- 
where is in the Lower Trias, while the oldest succeeding marine fauna 
is Upper Cretaceous. It is obvious, therefore, that neither the ances- 
tors nor the descendants of its species are found in the same area, but 
fortunately its stratigraphic position is fairly well determined in the 
much fuller Alaskan section. On the shores of Cook Inlet the Middle 
and Upper Jurassic are represented by about 10,000 feet of strata 
with at least three distinct marine faunas’ which are largely still 
undescribed. The strata have been almost equally divided into the 
1 Stanton and Martin, ‘Mesozoic Section on Cook Inlet and the Alaskan Penin- 
sula,” Geol. Soc. Am. Bull., Vol. XVI, pp. 391-410. 
