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 few, 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 alveole. 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- 
ever, shorter, as for example in the shrews. The molars and premolars 
vary 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, and 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 vertebree, as shown by the presence of floating ribs. They are distin- 
guished by the small development of the lateral apophysis. The vertebre 
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 vertebra, and they diminish in size backwards. The 
abdominal vertebre, on the contrary, increase in size backwards; they are 
easily distinguished from the vertebrae of the back by the absence of articu- 
599 
