GENERAL CHARACTERS. 635 



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

 sometimes restricted to early stages in life. The skin has 

 abundant sebaceous atid sudorific glands. In the female, 

 milk-giving or mammary gla?ids develop, as specialisations 

 of sebaceous gla?ids, 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 abdoininal 

 cavity, and is of great importance i?i respiration. 



All the important bones have distinct terminal ossifications 

 or epiphyses, absent, however, in the vertebrce of Afonotremes 

 and Sirenia. The centra of the vertebrce have generally flat 

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

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

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

 has nine ; and the pangolin (Manis), 'which has someti7nes 

 eight, — variations ivhich, it will be observed, are limited to the 

 two most old-fashioned orders of placeiital 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. 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 

 ivhich 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 — ivhich connect the drum 

 with the inner ear, that they correspond respectively to the 

 articular, quadrate, and cohcmella or hyo-7nandibular of 

 other Vertebrates. The otic bones fuse to form a compact 

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

 and palatines, separates the buccal cavity from the nasal 

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

 the premaxillce, maxillce, and mandible. 



Except in Alonotremes, the coracoid is represented by 

 a smcill process from the scapula, forming part of the 

 gletioid cavity in which the head of the humerus works, 

 b7it not reaching the sternum. The latter includes (a) a 



