236 



THE AGE OF MAMMALS 



Lower Rosebud beds, which lie farther east along the Pine Ridge of the 

 White River, speaks of the entire fauna as an outgrowth of the Oligocene 

 (White River and John Day faunas) containing no new or migrant elements.' 

 Among the most peculiar forms are Syndyoceras (a collateral successor of 

 Protoceras of the Leptauchcnia Zone) , a hypcrtragulid with an extraordinary 

 development of the paired horns on top of the skull (Fig. 121), and Steno~ 



mylus. a small and delicately 

 limbed cameloid abundantly 

 found in one locality (Fig. 118). 

 Oxydactylus (Fig. 144) is a long- 

 necked, or giraffoid camel, the 

 first of a very important Mio- 

 cene and Pliocene series. 



The hares, or leporids, are 

 represented by species which 

 cannot be separated from the 

 modern genus Lepus, although 

 the tooth pattern is primitive. 

 The EquidjE are represented 

 only by species with lateral 

 toes, typically reaching the 

 ground, with no vestige left of 

 the poUex. The oreodonts are 

 almost as abundant as in the 

 Oreodon beds of the White 

 River group. Especially re- 

 markable is the group contain- 

 ing three specimens of Pro- 

 mcrycochoerus, which is now 

 mounted in the Carnegie Mu- 

 seum; the animals were huddled 

 together as if overtaken by a 

 sandstorm or sudden fall of 

 volcanic ash. The canids range from the size of foxes (Nothocyon) up 

 to the larger and more predaceous dogs (Cynodesmus) , but include also 

 the short-faced dogs (Enhydrocyon) . A giant dog from the Lower Harri- 

 son was first defined as an amphicyon, but is now distinguished as Da- 

 phcenodon (Peterson). The peccaries, or dicotylids (Desmathyus) , are still 

 closely related to the Perchcerus type of the John Day. They are extremely 

 abundant and characteristic in this zone. 



In conclusion emphasis may be laid upon the fact that these plains 

 formations are difficult to correlate in point of time, that they may be 



1 Matthew, W. D., A Lower Miocene Fauna from South Dakota. Bull. Amer. Mus. 

 Nat. Hist, Vol. XXIII, 1907. 



Fig. 120. — Contemporary Upper Oligocene or 

 Lower Miocene mammals to same scale. By Charles 

 R. Knight. A. Syndyoceras, successor to Protoceras. 

 B. Phcnacocoelus, successor to Oreodon. A browser 

 or grazer. C. Promerycochaerus, precursor of Mery- 

 cochoerus. 



