14 



three groups, Hekbivora, Carnivora, and Insectiv- 

 ORA, which are again divisible into tribes and fami- 

 lies. Thus among Herbivora there are Root-eaters, 

 Grass-eaters, etc., with extinct forms which com- 

 bined the characters of several groups. The Marsu- 

 pals seem Kke a first essay of the higher mammals 

 which appeared later. The Didelphians and the Mon- 

 otremes constitute the Implacentals. 



The families Dromatheriidce and J'/n^idu/dcidcB, 

 which are Mesozoic mammals, are usually classed with 

 the Didelphians. They may possibly be neither 

 Ornithodelphians nor Didelphians, but, like these, dis- 

 tinct groups of generalized mammalian forms. 



As we are wholly unacquainted with their internal 

 organization, it is impossible to do more than conjec- 

 ture whether or not they were implacentals. Like 

 those comprehensive vertebrates, the viviparous sharks, 

 they may have included species with a rudimentary 

 placenta. It seems strongly probable that extinct 

 forms carried up the line unbroken from the earliest 

 vertebrates to the placental mammals. 



The MoNODELPHiANS, which are the placental mam- 

 mals, arise as a main stem from the implacental base 

 with a structure superior to these, carrying up, doubt- 

 less, characters derived from the earliest of vertebrates. 

 They appeared in force during the Tertiary period, 

 branching, in the Eocene, into their main divisions, 

 although the Edentates, which are the lowest in struc- 

 ture, did not appear until later. The extinct Eden- 

 tates belong to the later Tertiary and to the Quater- 

 nary Periods, and include the gigantic Megathniiuii , 

 the huge OUiptodon, and other fossil animals. The 



