XXV] CLASSIFICATION 7 17 



by the fact that the tritubercular molars lost their enamel late in 

 life, become at this period distinguished by the restriction of the 

 enamel to bands and the reduction of the incisors. Still higher in 

 the series Bats (Cheiroptera) make their appearance, little different 

 from what they are at present. 



In the horizon above this (Upper Eocene) the ancestors of Whales 

 are found, as the Archaeoceti (Zeuglodon) with well-developed nasal 

 bones, the nostrils placed about the middle of the snout, and with 

 double-rooted serrated molar teeth, derivable from the tritubercular 

 type by the development of additional cusps, all like the original 

 three being in the same line. True Carnivora distinguished by the 

 carnassials have likewise been by this time developed from the 

 Creodonta ; amongst the Ungulata the earliest forms of Camels and 

 of Tragulidae have appeared, as well as the forerunners of Elephants 

 and Sirenia. 



Once formed, Carnivora rapidly become differentiated, for in the 

 next period (Oligocene) Felidae and Viverridae had already appeared, 

 and contemporaneously with them the first Deer (Protoceratidae) 

 and the earliest Sirenia with visible hind-limbs (Halitherium). Still 

 higher the Elephants (Proboscideae) appear represented at first by 

 forms with both lower and upper tusks or even lower alone 

 (Mastodon and Dinotherium). At the same time the Deer first 

 appear with antlers and the Rhinoceros acquires a horn, and the 

 family of Bears (Ursidae) is commencing to be distinct from the 

 primitive Dog-like Carnivora, the gradual reduction in size of the 

 premolars, and of the carnassial marking the change. True Apes 

 (Anthropoidea) here succeed the Lemuroidea. 



In the next period (Miocene) the Giraffe (Samotkerium), Hyrax 

 and Orycteropus appear and so practically the whole group of 

 Mammalia has made its appearance, the remaining changes consisting 

 chiefly in the extinction of many forms either completely, or partially, 

 so that their representatives are now restricted to limited areas. It 

 will be noted how completely the geological evidence bears out 

 the idea of the central position of the group Insectivora among 

 Mammalia. 



The class Mammalia is divided as follows : 



Sub-class I. PROTOTHERIA. 



Mammalia which lay large eggs and in which the two oviducts 

 are completely separated, and there is a persistent cloaca. No 

 placenta. 



Ex. OrnitkorhynchuSj Echidna. 



