40 



GXiroEBOOK OF THE WESTERN UXITED STATES, 



from Carboniferous to Cretaceous; the rocks of the most prominent 



toward the north are those of the Casper formation 



ridge seen 



formed 



Permian) 



represent the Dakota sandstone of eastern Nebraska,* 



may 



Among the character- 



kind. They had a somewhat raore mod- 

 ern aspect than the animals that preceded 

 them, for the processes of evolution had 

 been active, and some of the primitive 

 animals of Eocene time had developed 

 into forms more nearly like those with 

 which we are familiar now. Others seem 

 to have left no descendants. Great num- 

 bers of Oligocene fossils have been found, 

 and the life of the time is probably better 

 known than that of any other epoch of the 

 Tertiary period 

 istic animals of this epoch were primitive 

 forms of rliinoccroses, peccaries, rumi- 

 nants, camels, insectivores, and opossums. 

 Some of the creodonta or flesh eaters of 

 Eocene time had developed into true 

 carnivores, including many forma of both 



The saber- 

 toothed cats which later developed into 



r 



the saber-toothed tiger, one of the most 

 formidable enemies of primitive man, first 

 appeared in the Oligocene. 



The horses whose history began with the 

 diminutive four-toed Eohippua continued 

 in the Oligocene, where they are repre- 

 i^nted by many three-toed forms which 

 were about as large as sheep. Iloglike 

 animals 



doglike and catlike animals. 



were rather 



and 



numerous, 



althou2:h manv of them were smaller than 

 the modern swine some of them were very 

 large. One of these, Archeotherium ingens 

 (see PI. VII, C, p. 41), was a formidable 

 beast with curious protuberances on its 

 head, the use of which is not known. 

 Rliinoce^oees similar to those now found 



in Africa and India lived in western 

 America, and other rhinoceros-like ani- 

 mals known as anymodonts were abun- 

 dant, but rhinoceroses did not reach their 

 culmination in America until the Pleisto- 

 cene epoch. 



In addition to these animals of more 

 modern appearance there were many that 

 were so unlike anything now living that 

 it is not possible to designate them by any 



common names. 



Among those are the 



animals of the protocerine group, of whose 

 history little is known. They seem to 

 have appeared suddenly in North Amer- 

 ica in Oligocene time and disappeared 

 from this continent dming the early pai't 

 of the Miocene. They were deerlike 

 creatures about the size of sheep. The 

 head of the male \xjl6 grotesquely orna- 

 mented with short bony protuberances 

 and large scimitar-like tusks. Each front 

 foot had four toes and each toe had a lioof 

 like that of a deer or antelope. The sup- 

 posed appearance of these curious animals 

 is indicated in the restoration of one of the 



F 



forms (Protocei'as celer) reproduced in 

 Plate VII, E , 



^ The table on page 41 shows the geologic 

 formations exposed in the \dcinity of the 

 Laramie Mountains near the Union 

 Pacific Railroad in the order of their ago, 

 the oldest at the bottom and the youngest 

 at the top. The position of these forma- 

 tions in the complete geologic time scale 

 may be ascertained by comparison with 

 the table on p. 2, 



