1 86 T. W. STANTON 



are the Mediterranean or Alpine, the Middle European, and the boreal 

 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 fauna. — 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. 



