192 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 lahiatus and 

 Prionotropis 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 



