TBKTIAKT MAMMALIA. 175 



little or no doubt, seeing what modifications of insectivore stmcture 

 are presented in the earliest deposits containing their remains, that 

 they appeared at a very much earlier epoch. The Tillodontia, 

 Creodonta, and Insectivora appear, as it were, simultaneously in 

 the Lower Eocene deposits, and if, therefore, they represent merely 

 modifications of one and the same structure, as is maintained by 

 Professor Cope, who has united the three groups into the one com- 

 prehensive order of the Bunotheria, then they must point to a com- 

 mon progenitor (foreshadowed in the Jurassic insectivorous marsu- 

 pials) removed far beyond the limits of the Tertiary period. Of 

 the three insectivore types here indicated, the true Insectivora, 

 which may be considered as the main or axial stem, have alone 

 survived to the present time. The Tillodontia and Creodonta both 

 became extinct before the middle of the Tertiary period, the latter, 

 however, by gradual modification passing off into the Carnivora, 

 whose earliest undoubted remains are to be found in the deposits 

 of Oligocene, or Miocene age. From the same group of the Insec- 

 tivora, although apparently at a somewhat later date than the Creo- 

 donta and Tillodontia, appear to have been descended the so-called 

 Prosimia, or primitive monkeys, the lemurs, whose earliest remains 

 occur in deposits of both Lower and Upper Eocene age ; and to these 

 last, again, is doubtless to be traced the direct line of ancestry of the 

 various types of true monkeys that at the present day inhabit the 

 earth's surface, and whose unquestionable traces are first met with in 

 deposits of Miocene age. The most important non-insectivore type 

 of Lower Eocene mammalian is the ungulate, whose remains, be- 

 longing to both the odd-toed and even-toed sub-orders, occur in 

 astonishing abundance, and argue very strongly in favor of a very 

 remote ancestory, one that may not impossibly carry us as far back 

 as the middle of the Mesozoic era. 



The progressive modifications of structure which can be traced 

 through the more generalised of the Eocene mammalian groups 

 results in greater and greater specialisation the further we advance 

 in the course of time, and hence, in the Miocene period we meet 

 with more of distinctly specialised (or isolated) groups than in the 

 period preceding. In addition to the recent orders that have been 

 enumerated as belonging to the Eocene period we have the Eden- 

 tata (represented by such gigantic forms as Macrotherium and 

 Ancylotherium, whose nearest relationship appears to have been 



