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 
Strenians, 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 ts, 
in most cases, prolonged into a tail. 
Fairs 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 vertebrae have 
generally flat or slightly rounded faces, and there are usually 
seven cervical vertebra.’ 
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 
life, of a single bone which works on the squamosal; the 
1Jn the Manatee there are, however, only six; the pangolin ands 
has sometimes eight; and it is often said that the two-toed sloth 
(Cholepus hoffmannt) 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, ¢.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. 
