132 THE AGE OF MAMMALS 



With the arrival of the new and retention of tlic old life, the Wind River 

 stands directly intermediate in position ; in fact the archaic and modernized 

 mammals are nearly evenly balanced, as shown in the following summary: 



Wind River Genera and Species 



Genera Species 



Archaic mammals 21 30 



Modernized mammals 22 36 



In this estimate, as before, the Insectivora are somewhat arbitrarily 

 placed with the anchaic, the Lemuroidea with the modernized forms. How- 

 ever reckoned, this balance between representatives of families destined to 

 become extinct and those destined to populate the earth is extremely 

 interesting. , Two families of creodonts (Arctocyonidse, Palaeonictidae) 

 are sparingly represented, and have possibly disappeared. No other fami- 

 lies or genera are known to have become extinct. 



Archaic mammals. — The chief surviving archaic mammals are the co- 

 ryphodonts, the condylarths, including Phenacodus, Ectocion, and Menis- 

 cotherium, three families of creodonts (Oxj^senidae, Hysenodontidae, Meson- 

 ychidae), the insectivores, the tillodonts, and the edentate tseniodonts. 

 Among the archaic Ungulata it is noteworthy that both Coryphodon and 

 Phenacodus are apparently diminished in number. It was supposed until 

 recently that they were' also dwindling in size, but this proves to be an 

 error, because large forms of l)oth animals were discovered by the American 

 Museum party of 1909, including a phenacodont as large as the P. primcevus 

 of the Wasatch. Undoubtedly the competition between the condylarths 

 and the smaller but better endowed horses, tapirs, titanotheres, and lophio- 

 donts was becoming very severe. It is extraordinarily interesting to find 

 the little Meniscotherium of the Wasatch again appearing in the Wind River. 

 There are three or four species of Coryphodon, including chiefly animals of 

 much smaller size than prevail in the Wasatch, and also one form of robust 

 size. 



Simultaneous with the decline of the coryphodonts, it is most interesting 

 to record the appearance of the first member (Bathyopsis) of the related 

 family (Uintatheriidse) of giant Amblypoda which are destined to become 

 the great quadrupeds of the Middle Eocene. For years only the jaw of 

 the type species (B. fissidens) was known, but a skull discovered by Olsen 

 of the American Museum party of 1909 shows that the animal possessed 

 a pair of nasal frontal horns above the orbits, although there are no horns 

 on the back of the skull, as in the Bridger successor, Uintatherium. 



Of the diprotodont, gnawing, or leaf -eating mammals, the small Wasatch 

 type (Esthonyx) persists, representing the order Tillodontia, which is also 

 destined to survive into Bridger times. Among the tseniodonts, or supposed 

 primitive Edentata, the large Wasatch type Calamodon survives and a 



