414 1 WSL ANTON 
or Russian, each characterized by different types of ammonites and 
other invertebrates. For example, the ammonite genera Lytoceras 
and Phylloceras are abundant in the Mediterranean province, occur 
sparingly in the Middle European, and are practically absent from 
the Russian Jura. Coral reefs and important limestones also are not 
found in the boreal Jurassic formations. 
In America there is no difficulty in recognizing a boreal fauna in 
the Upper Jurassic which, as we have just seen, temporarily extended 
far south in the Rocky Mountain region and at a later stage still 
farther south along the Pacific coast. It is like the Russian fauna 
in its essential features although it does contain the Mediterranean 
types Lytoceras and Phylloceras in Alaska. There is likewise no 
difficulty in recognizing a southern or Mexican fauna in which are 
commingled many of the types which in Europe are separated and 
considered characteristic of the Middle European and Mediterranean 
provinces. Finally the Mexican fauna received by way of the Pacific 
a few immigrants from the boreal fauna. 
Variations in the lithologic development are worthy of note. 
Limestones form a large proportion of the sediments in Mexico while 
they are relatively inconspicuous in all the areas where the boreal 
fauna is dominant. 
Jurassic (?) freshwater jauna.—The marine Jurassic beds through- 
out the Rocky Mountain region of the United States are immediately 
overlain by the continental freshwater or marsh deposits of the Morri- 
son formation which also extend south through Colorado into New 
Mexico beyond the limits of the marine Jurassic beds. Its large 
and varied dinosaur fauna was originally assigned to the Jurassic 
without question, but during the last few years some paleontologists 
have referred it to the Cretaceous. Its stratigraphic position is con- 
sistent with either reference as the interval otherwise unrepresented 
comprises a considerable part of each system. Its invertebrate fauna 
consists of several species of Unio, Vivipara, Planorbis, etc., all of 
modern freshwater types which do not assist in discriminating between 
Jurassic and Cretaceous. The fact that the Morrison is overlain 
by the Kootenai on the north and by the marginal deposits of 
the Comanche on the south tends to place it early in the transition 
interval. 
