GENERAL CHARACTERS OF MAMMALS. 669 



But it must be carefully noted that these orders are often linked by 

 extinct types. Thus, to take one instance only, it is believed by some 

 that the extinct Phenacodus has affinities with Ungulates, Carnivores, 

 and Lemurs. 



\\e may summarise our general classification thus : 



A MAN A 



and 

 MOX KEYS 



UNGU LATES CAR NIVORES 



BATS 



A 



LEM URS 



H 



RODENTS INSECTIVORES 



Y 



CETACEANS 



SIRENIA | EXTINCT SYNTHETIC TYPES | EDENTATAj 



II. MARSUPIALS 



n 



in 



I. MONOTREMES 



GENERAL CHARACTERS OF MAMMALS. 



All Mammals are quadrupeds, except the Cetaceans and 

 Sirenians, in which the hind-limbs have disappeared, leaving 

 at most internal vestiges. There is generally a distinct neck 

 between the head and the trunk, and the vertebral column is, 

 in most cases, prolonged into a tail. 



Hairs are never entirely absent. In most they form a thick 

 covering, but they are scanty in Sirenians and in the hippo- 

 potamus, and almost absent in Cetaceans, in which they are 

 sometimes restricted to early stages in life. The skin has 

 abundant sebaceous and sudorific glands. In the female, 

 milk-giving or mammary glands develop as specialisations 

 of sebaceous glands, except in Monotremes, where they are 

 nearer the sudorific type. 



A complete muscular partition or diaphragm separates the 

 chest cavity, containing the heart and lungs, from the abdominal 

 cavity, and is of great importance in respiration. 



The vertebrce and long bones have terminal ossifications or 

 epiphyses, absent or very rudimentary, however, in the vertebra 



