230 THE AGE OF MAMMALS 



and Lower Oligocene ischyromyicls of North America. Noteworthy also 

 is the first appearance of the geomyids, or pocket gophers, another rodent 

 family which is peculiar to North America. Besides these castorids, gophers, 

 sewellels, and sciurids, there are murids and leporids, the last family includ- 

 ing a species actually referred to the existing genus Lepus. 



The perissodactyl ungulates are now reduced from seven families to 

 the three existing families of Equidae, Tapiridae, and Rhinocerotidae, and the 

 extinct aberrant family of Chalicotheriidce. Among the rhinoceroses the dic- 

 eratheres are in a much more advanced stage of evolution than in the 

 Protoceras or Leptauchenia Zone, as they exhibit a pair of well-developed 

 horn cores upon the nasals (as in the D. minutum of Moissac, France), 

 and include animals of a considerable range in size. These pair-horned 

 rhinoceroses are especially numerous and characteristic. Aceratheres have 

 not been recognized in the Oregon region, although they were undoubtedly 

 present. Among the tapirs, Protapirus is similar to that found in the 

 Protoceras sandstones of the White River group, a single species, P. robus- 

 ius. The horses are numerous; they present a slight advance upon those 

 of the Protoceras sandstones, and are now chiefly referred to the stage 

 Miohippus, wliile one species (A. prccstans) from the Upper John Day is 

 very similar to the Lower Miocene Anchitherium of Europe. The chali- 

 cotheres are represented by three species in the Moropus stage of evolution. 



The Carnivora ^ are confined to the three families, canids, mustelids, 

 and felids. Of these the dogs still include Cynodictis (Merriam) beside a 

 great variety of more modern genera (Nothocyon, Mesocyon, Temnocyon, a 

 descendant of Daphcenus, Paradaphcenus, Enhydrocyoyi, Philotrox). This 

 is the most remarkable assemblage of canids known in any formation 

 in America. Beside the long-faced dogs (Temnocyon) there were short- 

 faced forms (Enhydrocyon, Philotrox), analogous to the Simocyon type of 

 Europe. Oligobunis was formerly considered a dog; it is now regarded as 

 a mustelid. The felids also indicate that during the deposition of the 

 Mitldle John Day the region included a country of open savannahs favor- 

 ing the running types. The mid-Oligocene separation observed by Matthew ^ 

 between (1) the Dinictis phylum, which is more truly feline, and (2) the 

 Hoplophoneus phylum, which is more truly machaerodont, is now indicated 

 by the light-limbed Nimravus, and the machaerodont Hoplophoneus, which 

 still survives. 



Of the Artiodactyla, five families are known, namely: entelodonts, 

 dicotylids, oreodonts, hypertragulids, and camelids. The entelodonts 

 begin to approximate the climax of their evolution, including animals of 

 massive size and robust limbs (E. imperator), and a considerable variety 



' Merriam, J. C, Carnivora from the Tertiary of the John Day Region. Univ. Cal. 

 Puhl., Bull. Dept. GeoL, Vol. V, no. 1, 1906, pp. 1-64. 



^ Matthew, W. D., Fossil Mammals of the Tertiary of Northeastern Colorado, 1901; pp. 

 387-394. 



