174 GEOLOGICAL DISTRIBUTION. 



(Zeuglodon), ungulates, both odd-toed and even-toed (among the 

 former the tapiroid Lophiodon and Paluaotherium, and other such 

 forms as Eohippus and Hyracotherium, which, through a series of 

 modified but closely-related types in the Miocene and Pliocene peri- 

 ods Anchitherium, Hipparion can be traced genetically to the 

 modern horse ; and among the latter the possible ancestors of some 

 of the modern deer, Xiphodon and Anoplotherium, and the suil- 

 line Anthracotherium and Palaeochoerus), bats, even of existing gene- 

 ra (Yespertilio, Vesperugo), lemurs, or lemurif orm insectivores (Ada- 

 pis, Necrolemur), and not impossibly also the true monkeys. But 

 while most of the forms found in these earlier Tertiary deposits are 

 referable to modern orders, there are others which would appear to 

 have no place in the classification laid down for living forms, and 

 which combine, in many respects, the characters of two or more 

 orders. Thus, it has been convenient to designate an order Ambly- 

 poda for a line of animals which, at the one extremity, stand near- 

 est in their relationship to the Proboscidea, or elephants, and at an- 

 other to the odd- toed ungulates. In it are comprised the Uinta- 

 theria, ponderous tusked-animals, rivalling or exceeding in size the 

 modern elephant, and the coryphodons, considerably smaller ani- 

 mals of a generalised type, the probable progenitors of the last. A 

 still earlier type is embodied in the Condylarthra (Phenacodus), 

 from the very base of the Eocene, which represent the most primi- 

 tive type of known ungulate animals, and which not impossibly 

 are derivatives of some preceding hoofed marsupial. Another order, 

 the Tillodontia, has been established for certain animal forms which, 

 in several respects, combine the characters of the insectivores, ro- 

 dents, and edentates; and, again, a fourth order, the Creodonta, 

 for forms that seem to hold a position intermediate between the 

 insectivores and carnivores, and not unlikely represent the ancestral 

 line of the latter. 



From the researches of paleontologists it would appear that 

 the primitive type of placental mammal is the insectivore, and 

 that from this original type have descended, by gradual modifica- 

 tion, most of the varied forms that now people the surface of the 

 earth, and those w r hose remains lie buried in the deposits of the 

 Tertiary period. At what precise period in the earth's history the 

 Insectivora first appeared it is impossible to say, for, although no 

 remains occur in any deposits antedating the Eocene, there can be 



