420 T. W. STANTON 
Niobrara is represented by shale, and is not lithologically separable 
from the Benton. ‘The fauna is here correspondingly modified and 
a number of Niobrara and Austin species are associated with an 
assemblage of other forms peculiar to the region, together with a 
few that show closer relationship with the Benton fauna. 
“A horizon near the top of the Benton in Texas, New Mexico, and 
southern Utah is characterized by an abundance of ammonites 
belonging to the genus Metoicoceras Hyatt, formerly referred to 
Buchiceras, together with a number of other forms not known else- 
where. A littoral facies of the Benton fauna is developed in Utah 
and western Wyoming, and locally in southern Colorado, associated 
with sandstones and, except in Colorado, with coal-beds. 
These local or temporary differences in the Colorado fauna may 
be attributed to differences in depth, in proximity to the shore, and 
possibly to variations in climate conditioned on ocean currents. With 
a shallow sea and an open connection with the Arctic the southern 
local facies in the Benton and the Niobrara would probably correspond 
with the area directly influenced by the equatorial or gulf current. 
Certain important forms, however, like Inoceramus labiatus and 
Prionotro pis woolgari are distributed throughout the entire area. 
In the Athabasca region of northwestern Canada a_ peculiar 
ammonite fauna has been described from the Peace River sandstone, 
and the Loon River and Clearwater shales, all of which are referred 
to the Colorado group; but the question of their age and relationship 
should be left open until the geology. and paleontology of the region 
are known more in detail. It has been suggested that they may be 
older than Colorado. 
Montana fauna.—From New Mexico northward the Montana 
group has nearly the same distribution and extent as the Colorado 
group. It varies greatly in character, from all marine in some areas 
to largely brackish and freshwater deposits in others, and its faunas 
are correspondingly differentiated. A considerable element of its 
marine fauna is evidently derived directly from the Colorado fauna 
but a large proportion of it is apparently composed of immigrants 
from other areas, probably in part Arctic and in part Atlantic. In the 
north a littoral facies associated with sandstones and a deeper-water 
facies (the Pierre fauna) in shales may be distinguished. The littoral 
