PART III.
DENTAL SYSTEM OF MAMMALS.
C H A P T E R I.
GENERAL CHARACTERS OP THE TEETH OF MAMMALS.
124. The class Mammalia, like that of Pisces and Reptilia,
includes a few species which are entirely devoid of teeth; these are
the Ant-eaters, forming, when clothed with hair, the genus Myrme-
cophaga; when defended by scales, the genus Manis; and when
armed with spines, the genus Echidna. A few Mammals , have the
jaws provided with horny substitutes for teeth, as the Whalebone-
whales (Balcena and Balamoptera), and the Ornithorhynchus; in
the rest of the class true teeth are present. In the Feline tribe
the epithelium of the tongue is thickened at the fore-part of its
dorsum, and invests the papillae there with hard sheaths, like
prickles, which are analogous to the lingual teeth of certain Fishes
and Batrachians. The back part of the dorsum of the tongue in the
Echidna is provided with a plate of horny denticles, which bruise
its food against the hard and prickly epithelium covering the palate.
Horny processes, analogous to the palatal teeth of Fishes and Reptiles,
are likewise developed upon the roof of the mouth of the great
Bottle-nose Dolphin, thence termed Hyperoodon by Lacepede.
125. Number. In the last-named Cetacean, the true teeth are
two in number, whence its specific name, bidens: the Narwhal
likewise has but two teeth, both of which are concealed in the substance
of the jaws m the female, whilst only one is ordinarily visible
in the male: but this grows to an unusual length. The Delphinus
gnseus has five teeth on each side of the lower jaw; but they soon
become reduced to two on each side. Amongst the Marsupial
animals, the genus Tarsipes is remarkable for the paucity as well
as minuteness of its teeth. The Elephant has never more than
one entire molar, or parts of two, in use on each side of the
upper and lower jaws, to which are added two tusks, more or less
developed in the intermaxillary bones. Some Rodents, as the
Australian Water-rats, (.Hydromys), have two grinders on each side
of both jaws; which, added to the four cutting teeth in front,
make twelve in all: the common number of teeth in this order
is twenty; but the Hares and Rabbits have twenty-eight teeth.
The Sloth has eighteen teeth. The number of teeth, thirty-two,
which characterises Man, the Apes of the old world, and the true Ruminants
is the average one of the Class Mammalia. The examples
of excessive number of teeth are presented, in the order Bruta,
by the Priodont Armadillo, which has ninety-eight teeth; and, in
the Cetaceous Order, by the Cachalot, which has upwards of sixty
teeth, though most of them are confined to the lower jaw,—by the
common Porpoise, which has between eighty and ninety teeth,—
by the Gangetic Dolphin, which has one hundred and twenty teeth,
and by the true Dolphins (Delphinus), which have from one hundred
to one hundred and ninety teeth, yielding the maximum number
in the class Mammalia.
126. Form.—Where the teeth are in excessive number, as in
the species above cited, they are small, equal, or sub-equal, and of
a simple conical form: pointed and slightly recurved in the common
Dolphin; with a broad and flattened base in the gangetic Dolphin (Inia);
with the crown compressed, and broadest in the Porpoise ; compressed,
but truncate, and equal with the fang in the Priodon. The simple
dentition of the smaller Armadillos, of the Orycterope, and of the
three-toed Sloth presents a difference in the size, but little variety
in the shape of the teeth, which are sub-cylindrical with broad
triturating surfaces: in the two-toed Sloth, the two anterior teeth
of the upper jaw are longer and larger than the rest, and adapted
for piercing and tearing. In almost all the other Mammalia
particular teeth have special forms for special uses ; thus, the
front-teeth, from being commonly adapted to effect the first coarse
division of the food, have been called cutters or incisors, and