4 General Characters. MAMMALFA. General Characters. 
margins. The two halves of the lower jaw consist 
each of a single bone ; they are united in front either 
by a cartilage or by a suture, or sometimes, as in 
man, the two sides of the jaw are completely amal- 
gamated so as to form one bony piece. The lower jaw 
in the Mammalia is articulated directly to the skull, 
without the intervention of any other movable bone. 
The jaws, as already intimated, are furnished with 
teeth, and these exhibit a great diversity in their form 
and structure. Tliey are always implanted in sockets 
of the jaws, and these are lined by a delicate mem- 
brane, so that the teeth are never anchylosed or com- 
pletely united to the bone of the jaws. The teeth 
consist of a hard substance called dentine,^ defended by 
a coating of enamel, and covered by a layer of a third 
substance called cement. The latter is very thin on the 
crown or exposed portion of the tooth in man and 
many animals, which have teeth similar to those of the 
human species ; but in the teeth of many herbivorous 
mammals the cement acquires a great development, 
and vertical folds of this substance and enamel penetrate 
the dentine of the crown, thus giving rise, as the teeth 
are worn away, to an uneven surface eminently adapted 
to the comminution of tough vegetable matters. 
A few species are entirely destitute of teeth ; in 
others a few of the teeth are wanting, or some of 
them undergo peculiar modifications to adapt them to 
particular purposes. But in the majority w’e find four 
different sets of teeth called respectively the fnczsors, 
or cutting teeth; \h.e canines ; the premolars, or false 
molars ; and the molars, or grinders. The incisors or 
cutting teeth are inserted in the interma.xillary bones 
in the upper jaw, and occupy the corresponding place 
in the lower one. Their number varies from two to 
ten, and their form is also subject to much diversity ; 
but they are usually flattened transversely, so as to 
form a cutting edge across the front of each jaw. 
The canines, so called from their large size in the 
dog, are also veiy large in all carnivorous mammals. 
In the human subject the upper ones are frequently 
called eye-teeth, from their being placed directly 
beneath the eyes. Of the canines we find one on each 
side in each jaw ; the upper ones are inserted at the 
anterior angles of the maxillary bones, and the lower 
ones in a corresponding position in the lower jaw. When 
most largely developed, they form long, curved, conical, 
acute teeth, capable of inflicting the most serious 
wounds. 
The premolars, which are usually three or four in 
number on each side, are generally separated by a" 
short interval from the canines, which they frequently 
resemble in having only a single root ; their crown is 
usually broad and tubercular or ridged, in a manner 
more or less resembling that of the true molars. The 
latter, of which there are also commonly three or four 
on each side, are the largest and strongest of all the 
teeth, and are implanted in the jaws by two or more 
roots, a character peculiar to the Mammalia, and one 
which is often of the greatest importance to the 
palseontologist in determining the nature of those fossil 
remains by which a certain light has been thrown upon 
the former history of our planet. The molars, of all 
the teeth, are those which appear to undergo the 
greatest amount of modification to fit them to the 
habits and food of the animals. In the carnivorous 
forms we find them furnished with sharp cutting edges, 
and fitting together like the blades of a pair of scissors ; 
in those which prey principally upon insects, whose 
hard and slippery armour renders them rather difficult 
to be disposed of, the molars are furnished with a 
double row of sharp points, from which even the hard- 
est beetle could not find it easy to escape ; in those 
which, like the monkeys and our own species, feed 
upon fruits or upon a mixed diet of soft animal and 
vegetable substances, the crowns of the molars are of a 
more or less cubical form, with the surface divided into 
several blunt tubercles by furrows which traverse it in 
different directions ; and lastly, the strictly herbivorous 
species usually present an intermixture or alternation 
of the three substances of which the teeth are com- 
posed, such as produces a series of ridges upon their 
surface, as they are gradually worn down during the 
trituration of the food. 
The teeth are produced from a pulpy germ or matrix 
contained within the jaw, and in the majority of the 
Mammalia the activity of this germ continues after it 
has served for the formation of the series of teeth first 
produced. These, which are commonly known as the 
milk-teeth, are shed at a certain period of life, when 
their places are taken by new teeth adapted to the 
increased size of the jaw. The milk-teeth include the 
incisors, the canines, and three or four molars on each 
side ; the two former groups are replaced by new 
incisors and canines; the deciduous molars are shed 
to make room for the premolars, whilst the true molars 
are produced later than the other teeth, and are never 
changed. The teeth of the Mammalia are never shed 
more than once; but, in some forms, the formative 
pulps of some of the permanent teeth continue in 
activity during the whole life of the animals, and thus 
the teeth are constantly growing at the root. As these 
modifications of the teeth are usually characteristic 
of certain orders of Mammalia, they will be more 
particularly referred to hereafter, when the beautiful 
adaptation of their structure to the habits of the ani- 
mals will be more clearly seen. 
The general structure of the skeleton will not detain 
us long, as it nearly agrees with that already described 
(pp. 1, 2), as the most perfect development of the verte- 
brate type. The vertebral column, or back -bone, as it 
is usually termed, is divided into several regions, as has 
been already stated : these are called the cervical, 
dorsal, lumbar, and sacral regions, or the regions of the 
neck, back, loins, and sacrum ; and the continuation of 
the vertebral column into the tail, when this exists, 
constitutes the caudal region. The same names are 
applied to the vertebrae composing each region. 
Of the cervical vertebrae there are almost invariably 
seven ; and this is the only region of the body in which 
the number of vertebrae is at all constant.* Whatever 
may be the length of the neck in these animals, the 
number of the vertebrae is the same ; the short neck 
* The only exceptions to this rule are presented by the 
Sloths, in which the neck contains eight or nine vertebrse : 
and by the Southern Manatee {Manatus australis], which 
us\ially has only six cervical vertebrje. 
