GENERAL CHARACTERS. 635 



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. 



All the important bones have distinct terminal ossifications 

 or epiphyses, absent, however, in the vertebra of Monotremes 

 and Sirenia. The centra of the vertebrce have generally flat 

 faces, and there are seven cervical vertebra, except in the 

 manatee and the two-toed sloth (Cholcepus hoffmanni), which 

 have six ; the three-toed sloth (Bradypus tridactylus), which 

 has nine ; and the pangolin (Manis), which has sometimes 

 eight, variations ivhich, 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 tivo occipital condyles, 

 as in Amphibians. 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. 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 has disappeared, or has been shunted 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, quadrate, and columella or hyo-mandibular of 

 other Vertebrates. The otic bones fuse to form a compact 

 periotic. A bony palate, formed from premaxillce, maxilla, 

 and palatines, separates the buccal cavity from the nasal 

 passages. In most cases there are teeth, borne in sockets by 

 the premaxillce, maxilla, and mandible. 



Except in Monotremes, the coracoid is represented by 

 a small process from the scapula, forming part of the 

 glenoid cavity in which the head of the humerus works, 

 but not reaching the sternum. The latter includes (a) a 



