292 



THE AGE OF MAMMALS 



Fauna. — The conspectus of the fauna exhibits at once its wide 

 departure from the Upper Ohgocene and its successive Lower Miocene 

 stages as above described. 



The reported occurrence of edentates of the 

 gravigrade type is especially interesting because 

 these animals were supposed to have first in- 

 vaded North America in Middle Pliocene times. 

 It must be remembered that the occurrence of 

 armadillo-like forms intheBridger (p. 164) points 

 to the possibility that the edentates were resident 

 in certain parts of North America from early 

 times. 



The occurrence of gravigrade sloths in the 

 Miocene of the northwest seems consistent with 

 Scharff's theory that North and South America 

 were at times connected through Lower Cali- 

 fornia and western South America.^ But this 

 theory is inconsistent with the fact that other 

 animals did not pass south or north. 



The Proboscidea are relatively little known, 

 being represented by two species {T. proavus, T. 

 hrevidens), neither of which has the long, narrow 

 grinding teeth characteristic of T. angustidens of 

 the Lower Miocene of Europe. Of the rhinoce- 

 roses the species Teleoceras medicornutus, dis- 

 covered in the Pawnee Creek region of Colorado, 

 is a remarkably close successor of the T. aurelia- 

 nense of the Lower Miocene of France, because 

 both possess beside the terminal nasal horn a ru- 

 dimentary frontal horn. This is one of the most 

 brilliant illustrations of the migration theory 

 between the New and Old Worlds. Of the 

 aceratherine rhinoceroses, which are best known 

 in the plains region of Colorado, the type of 

 Cope's genus Aphelops {A. megalodus) occurs at this level. 



Of the browsing, or leaf-eating fauna, we find three important examples 

 among the horses, namely, Hypohippus of Oregon and Colorado, a supposed 

 forest-living horse with short-crested teeth and persistent tridactyl feet, 

 Archceohippus of the Mascall, Oregon, a small animal with teeth resembling 

 those of the Oligocene Mesohippus, but distinguished by two very large 

 preorbital pits; and the short-crowned Parahippus {=Desmathippus Scott), 

 in which, although the molars are brachyodont, a fine deposit of cement 



^ Scharff, R. F., On an Early Tertiary Land-connection between North and South Amer- 

 ica. Amer. Natural, Vol. XLIII, Sept. 1909, pp. 513-531. 



Proboscideans 



Trilophodon 

 Rhinoceroses 



Teleocerine 



Aceratherine 

 Horses 



Merychippus stage 



Hypohippus stage 

 Tapirs 



Chalicotheres 

 Peccaries 

 Oreodonts 



Ticholeptus stage 

 Camelids 



Protolabis stage 



Alticamelus stage 

 Palaeomerycines 



Dromomeryx 

 Merycodontids 



Merycodus 

 Edentates 



Megalonychids 

 Canids 



Amphicyonids 

 Mustelids 



Otters 

 Felids 



Pseudmlurus 

 Mylagaulids 



