MAMMALIA. 395 



elephant ; and of the upper, prehensile lip of the rhinoceros. The upper jaw 

 is always immovable, and united to the skull, whilst the lower moves 

 vertically against it, the latter always possessing two simple articulations 

 placed at right angles in the higher groups. 



The teeth, which are wanting in some i'ew, vary very much in their 

 number and their shape, and when they exist they are confined to the jaws 

 upon which they rest, implanted into alveolae. They are of four kinds, 

 incisors, canines, molars, and premolars, which sometimes exist together, 

 whilst sometimes only two kinds, or even only one is met with. Their 

 relative position is invariable and well known, and their form very charac- 

 teristic. The incisors are generally chisel shaped, sharp, and straight, 

 seldom curved, and always prominent among the others, occurring in vari- 

 able number, and inserted above in the premaxillary, and below on the 

 symphyses of the lower jaw ; in some genera, completely wanting. 

 The canines, still oftener absent, are acute, with a conical crown and a 

 single root, more or less curved, one in each half of the jaw, behind the 

 incisors ; they are often much larger than the other teeth ; sometimes, how- 

 *^ver, shorter, as for example in the shrews. The molars and premolars 

 % ary greatly, according to the nature of the food. In the Carnivora proper 

 they have a compressed and cutting crown ; they are compressed, again, 

 but tuberculous, in the beasts of prey feeding also upon vegetable matter ; 

 finally, they are sometimes flat, but usually furnished with enamelled ridges 

 in all those mammals which feed chiefly upon plants or vegetable sub- 

 stances. They are generally provided with several roots. In the whales, 

 the teeth in the upper jaw are replaced by the whalebones, which are 

 elongated, falcate, elastic, and flexible plates, their points directed down- 

 wards, provided at their inner extremities with innumerable elongated and 

 loose threads of the same substance as the whalebone itself. Ornithorhynchus 

 instead of teeth has a pair of horny tubercles, ^nA Echidna is provided on 

 the palate with several rows of spines directed backwards. 



Every bone composing the skull is united to its neighbor by intimate 

 suture, and sooner or later is soldered to it, so as to form a continuous 

 cavity for the brain. The skull articulates to the vertebral column, by 

 means of two condyles, with the atlas or first vertebra of the neck. The 

 articulation takes place below the great posterior opening through which 

 the brain passes into the spinal canal. The lateral motion of the head 

 does not take place upon the first vertebra, being performed by the first ver- 

 tebra upon the second. The neck, whatever be its length, consists of seven 

 vertebrae. In the supposed exception, the sloth, which appears to have nine, 

 we find, on careful examination, that the two last are really the two anterior 

 dorsal vertebrae, as shown by the presence of floating ribs. They are distin- 

 guished by the small development of the lateral apophysis. The vertebrae 

 of the back, to which ribs are always attached, vary greatly in number, but 

 are always more numerous than the abdominal ones ; their body is stouter 

 than that of the neck vertebrae, and they diminish in size backwards. The 

 abdominal vertebrae, on the contrary, increase in size backwards ; they are 

 easily distinguished from the vertebrae of the back by the absence of articu- 



599 



