GENERAL CHARACTERS OF MAMMALS. 527 



General Characters. 



Form. — All Mammals are quadrupeds, except the Ceta- 

 ceans and Sirenians, in which the hind-limbs have dis- 

 appeared, leaving at most only internal vestiges. In most 

 there is a distinct neck between the head and the trunk, 

 and the vertebral column is usually prolonged into a tail. 



Skin. — Hairs are never entirely absent. Usually they 

 form a thick covering for the skin, but they are scanty in 

 Sirenians and in the hippopotamus, and almost absent in 

 Cetaceans, in which they are sometimes restricted to very 

 early stages in life. The skin has abundant sebaceous and 

 sudorific glands. In the female, milk-giving or mammary 

 glands develop, apparently as specialisations of sebaceous 

 glands, except in Monotremes, in which they seem nearer 

 to the sudorific type. 



Muscular System. — A complete muscular partition or 

 diaphragm separates the cavity containing the heart and 

 lungs from the abdominal cavity, and as it alters the size of 

 the chest cavity, it is of great importance in respiration. 



Skeletal System. — All the important bones have distinct 

 terminal ossifications or epiphyses, absent, however, in the 

 vertebrae of Monotremes and Sirenia. The centra of the 

 vertebrae have generally flat faces, and there are always seven 

 cervical vertebrae, except in the manatee and the two-toed 

 sloth (Cholcepus liqffmanm), vihich have six; the three-toed 

 sloth {Bradypus tridactylus), which has nine; and in the 

 pangolin (Manis)., which has sometimes eight, — variations 

 which, it will be observed, are limited to the two most old- 

 fashioned orders of placental Mammals. 



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 quadrate which intervenes in Sauropsida having dis- 

 appeared, or been shunted out of place to become one of 

 the ear-ossicles. For it is one theory of the three ossicles — 

 malleus, incus, and stapes — which connect the drum with the 

 inner ear, that they correspond respectively to the articular, 



