IV.] LEMUROIDS. 155 



With the Puerco and Cernaysian faunas we take leave of the 

 Secondary multituberculates, and as we ascend in 



. . ... . Lemuroids. 



the Tertiary series we find a gradual and progressive 

 modification of the eutherian mammals towards the modern types. 

 In the ungulates especially the modification displays itself in the 

 more complex structure of the molar teeth, and in the reduction of 

 the number of toes ; the culmination of the latter line of develop- 

 ment being reached in the modern horses among the perissodactyle 

 section of the order, and in the ruminants among the artiodactyles. 

 As regards the molar teeth, the chief features are a lengthening of 

 the crowns in the more specialised later forms, accompanied by 

 complex infoldings of the surface of the crown and sides, whereby 

 the short-crowned, or brachydont type, as exemplified in the tapirs, 

 has developed into the tall, or hypsodont type characteristic of the 

 horses. Instead, however, of tracing the succession of the various 

 faunas, it will suit our present purpose better to refer to the 

 distribution of the more widely spread groups which are either 

 characteristic of Arctogsea as a whole, or which were common to 

 that realm together with Neogsea during the Plistocene period. 



The lemuroids, which are at present unknown beyond the 

 limits of this realm, are first met with in the Puerco beds, where 

 they are represented by Indrodon, and it is probable that the 

 Cernaysian mammal described as Plesiadapis (of which the upper 

 cheek-teeth are shown in the annexed figure) belongs to the same 



FlG. 31. THE RIGHT UPPER CHEEK-TEETH OF Plesiadapis : 



p. premolars, m. molars. 



group. It will be observed that in the latter genus the molars are 

 of the tritubercular type; the same being the case in Anapto- 

 morphus of the lower or Wahsatch Eocene of America. In other 

 forms, however, as in Hyopsodus and Pelycodus of the lower Eocene 

 of North America, and probably also of the European Eocene, as 

 well as in Microchcerus of the Oligocene of France and England, 

 the upper molar teeth have squared crowns, and thus approximate 



