OLIGOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 219 



tory meatus, which is phiced much lower on the sides of the head than in 

 the pigs or peccaries. In Mills' restoration the tuberosities on the lower 

 surface of the jaw represent a mechanism for muscular attachment. 





Middle Oligocene, Oreodon Zone or Brule Clays, 

 Stampian Stage 



Geology. — Immediately overlying the Titanotherium Zone in the White 

 River Group of the Great Badlands is the Oreodon Zone, or Brule Clays, 

 about 270 feet in thickness, and broadly distinguished as of Middle Oligo- 

 cene age. While the Titano- 

 therium beds are of colder, 

 grayish tint, these Oreodon 

 layers are warmer buffs and 

 pinks, and, as shown in Fig. 98, 

 are generally distinguished by 

 long, horizontal lines or bands 

 of similar color, which indicate 

 the frecjuent recurrence of over- 

 flow or Stillwater conditions. 

 These horizontal bands are an 

 indication of aqueous rather 

 than seolian deposition. Such 

 bands are seldom seen in the 

 Titanotherium Zone of South 

 Dakota, although observed in 

 Titanotherium beds elsewhere. 

 The Oreodon beds are divided 

 by the so-called ' nodular 

 la^^ers' (Fig. 96) into 'Lower,' 

 ' Middle,' and ' Upper,' each 

 with a more or less specific 

 fauna, and affording a series of 

 transitions, including arrivals 



and departures of animals of different kinds, similar to those which are 

 recorded in the Eocene levels of the Bridger. Thus the Oreodon Zone 

 alone represents a vast interval of geologic time. 



The lower Oreodon Zone is abruptly traversed by the 'Metamynodon 

 Sandstones' (Fig. 105), coarse river channel deposits, cross-bedded sand- 

 stones of greenish and brownish color, full of pebbles, containing especially 

 the amphibious rhinoceroses {Metamynodon) and other lowland forms. 

 In general, as first observed by Matthew, there is a sharp distinction be- 

 tween the fluviatile and river-border fauna contained in these sandstones 



Fig. 107. — MetainyuuilDii sandstuiu-s, river 

 channel beds in the Upper Oreodon layer.s of South 

 Dakota. Photograph by American Museum of 

 Natural History, 1894. 



