Ii6 GEOGRAPHICAL AND GEOLOGICAL DISTRIBUTION 



period very intimately connected. A marked feature of these 

 early Eutherians is the prevalency of trituberculism in the dentition, 

 not less noteworthy being the frequent occurrence of pentadactylism 

 in the feet, while many of the individual bones were devoid of the 

 grooves and ridges found in those of later types. By the time 

 that we reach the upper division of the Eocene period, such as the 

 horizon of the well-known gypsum of the Paris basin, nearly all the 

 chief groups of mammals had become clearly differentiated from 

 one another, although their representatives were usually more 

 generalised than their existing allies. From this date to the later 

 geological periods there is a gradual approximation to the types of 

 mammalian life existing at the present day. 



In addition to the features of trituberculism and pentadactyl- 

 ism so characteristic of the oldest known Eutherians, we may notice 

 some other points in connection with the earlier types. Thus the 

 older Tertiary mammals, as we have already stated, had relatively 

 smaller and simpler brains than the later types, so that a gradual 

 evolution in this respect may be traced from the Eocene to the 

 Pleistocene. Again, there is a great tendency among the Eocene 

 forms to a retention of the typical Eutherian dental formula noticed 

 on page 25, and also to the absence of an interval, or diastema, in 

 the dental series. Concomitantly with this feature we may notice 

 the short crowns and simpler structure of the molar teeth of the 

 earlier Ungulates as compared with those of to-day, of which details 

 will be given in a later chapter. Another instance of the more 

 generalised characters of the earlier mammals is afforded by the 

 absence or slight development of horns, antlers, and tusks among 

 the Ungulata. Thus the earlier Rhinoceroses were hornless, and 

 the Deer either without antlers or with antlers of a very simple 

 kind, while the ,male Swine were not furnished with the formidable 

 tusks of the existing Wild Boars. Finally, all, or nearly all of the 

 mammals, from the lowest Eocene of Rheims present the pecu- 

 liarity of having a vertical perforation in the astragalus. 



The intimate connection existing during the Middle Tertiary 

 between many families of mammals now widely distinguished from 

 one another may be more conveniently noted when we come to the 

 consideration of the families in question. 



