116 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- 
larity 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. 
