296 ORIGIN AND DISTRIBUTION OF MAMMALS 



certain incisors and of the canines, and among the ganodonts in the develojDment 

 of hypsodont, rootless teeth. The creodonts, however, which originally have 

 tri tubercular, i.e. tuberculo-sectorial, molars, usually specialise so that on the 

 upper's a long cutting metastyle arises and the protocone becomes reduced, and 

 on the lowers the metaconid often disappears and the talonid becomes notice- 

 ably reduced ; the protoconid and paraconid may also become converted into 

 a long cutting surface. At times the number of molars becomes reduced, and 

 in the case of the parent forms of the true carnivores a certain reduction in 

 the rear molars occurs. As regards the extremities, they are only known 

 in a few cases among European mammals, but the specimens which have been 

 found show that the reduction of the lateral digits took place early in the 

 developmental history not only of the artiodactyls but also of the perissodactyls. 

 The third and fourth digits assumed the principal function in the case of the 

 former, but only the third in the case of the latter, though not infrequently 

 the manus retains four and the pes three digits. Even the creodonts sometimes 

 lose the first digit. 



The marine mammals also appear for the first time in the Eocene. The 

 whales are only represented by the family Zeuglodontidae, but they are rather 

 widely distributed already in the southern part of North America, in Europe, 

 Egypt and on the island of New Zealand. The Sirenia are found in the West 

 Indies, Europe and Egypt. The former very probably may be traced back to 

 the creodonts, the latter are derived from the same form as the Proboscidea. 

 Both doubtless developed from terrestrial placentals. 



Oligocene. 



The Oligocene in North America exhibits a much sharper division into 

 successive faunas, and at the same time a considerably greater richness in forms 

 than in Europe. The White River beds of Nebraska, Dakota, Montana, 

 Colorado and Wyoming are readily divisible into three horizons. The lowest, 

 as well as the Tiianotherium bed, which occurs in the Cypress Hills, Canada, 

 is characterised by the gigantic Titanotheriidae, a family of perissodactyls, 

 and by insectivores resembling the Chrysochlmidae ; the middle, by the abun- 

 dance of. the Agriochoeridae and the richness in insectivores and Dideliihyidae ; 

 the uppermost, by the genera Leptauchenia and Frotoceras. In the latter 

 horizon, on the other hand, the Hyaenodontidae, which, with Elotherium, 

 Anthracotherium, Ancodus, and the pig Perchoerus, represent a European faunal 

 element, and the Leptktklae among insectivores are already completely extinct. 

 Common to all divisions of the White River Oligocene are Canidae — Amjjhi- 

 cyoninae and Cynodictinae, Mustelidae, primitive Felidae — Dinidis, Iloplophonens, 

 numerous rodents — especially CaMoridae, Ischyromyinae, and Leporidae, among 

 perissodactyls the forerunners of the horses — Mesohippus, tapirs, and rhinoceroses. 

 Nevertheless, of the tapirs Colodon, and of the rhinoceroses Hyracodon and 

 Metamynodon, are restricted to the older horizons, while Protapinis first appears 

 somewhat later, and Aceratherium on the contrary ranges throughout. the Oligo- 

 cene. In addition to the Camelidae and Agriochoeridae, which pass upwards 

 from the Uinta beds and now exhibit striking advances in their organisation 

 in the genera Poehrotherium, Agriochoems, Oreodon, and Leptauchenia, there are 

 here among selenodonts the small Hypertragulid.ae and, finally, Proloceras ; 



