694 MAMMALIA. 



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. 



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 vertebra and long bones have terminal ossifications or 

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

 of Monotremes and Sirenia. The centra of the vertebra have 

 generally flat or slightly rounded faces, and there are usually 

 seven cervical vertebral 



The bones of the skull are firmly united by sutures, which 

 generally persist. Only the lower jaw, the ear ossicles, and 

 the hyoid are movable. There are two occipital condyles, as in 

 Amphibians? The lower jaw on each side consists, in adult 

 tif e > f a single bone which works on the squamosal ; the 



1 In the Manatee there are, however, only six ; the pangolin Manis 

 has sometimes eight ; and it is often said that the two-toed sloth 

 (Cholaepus hoffmanni) has only six, and the three-toed sloth (Bradypus 

 tridactylus) nine ; but in the case of the sloths there is apparently con- 

 siderable variation. It will be noticed that these deviations from type 

 occur only in the case of the two most old-fashioned orders of Eutherian 

 Mammals. 



2 It may be noted, however, that for various reasons, e.g. that some 

 Birds and Reptiles are not very clearly single-condyled, morphologists 

 no longer attach so much importance to this character as they once did. 



