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OF THE GRINDERS. 89

The Enamel covers the bodies of these Teeth pretty equally
all round.
The first Grinder is somewhat larger and stronger than the

Hedgehogs (Eclrinops and Ericulus) resemble the Chrysochlore in the
more simple character of the molars, which are compressed antero-pos-
teriorly. Each upper molar presents two outer cusps and one inner,
each lower molar one outer and two inner. In both sub-genera, the pre-
molars (l\ V.) have simple crowns. The molars in Echinops are \\ %\
,
in Ericulus | \ £[ . The Tenrecs or Tailless Hedgehogs of Madagascar,
resemble, in the general character of the molar dentition, the last-named
genera. The first premolar above is compressed, unicuspid, with a hinder
talon, and implanted by two fangs ; the second is tricuspid, with three
fangs. The three first true molars are tricuspid and three-fanged ; the
last, which is smaller, has two fangs. In the lower jaw, all the molars
have two fangs. It is to be noticed, that the coating of enamel on the
teeth of these small insectivores is of great thickness when compared
with the body of dentine, a provision for the durability of that cuspidate
conformation, which is admirably adapted for cracking and crushing the
hard or tough coverings of their insect prey.
Order Cheiroptera. The molar series in the bats consists of premolars
and true molars, it never exceeds f ; % ; . The true molars are generally
bristled with sharp cusps. In the large frugivorous bats (Pteropus), the
true molars have flat crowns. True molars are absent in the Suctorial
or Vampire Bats {Desmodus). Two premolars are present on each side
in the upper jaw : they are very small teeth ; each has a simple, com-
pressed, conical crown, with one fang. In the lower jaw, there are three
premolars on each side ; the two first resemble the upper, but the third
has a larger, compressed, bilobed crown, and is implanted by two fangs.
In the Rodentia, the molars are few in number, obliquely placed, the
series on each side converging anteriorly in both jaws, and obliquely
worn. The variations in the form and structure of these teeth in this
order are so numerous as to typify almost all the modifications which
they present in the omnivorous and herbivorous genera of other orders.
In some Rodents, which subsist on mixed food, and display, as in the
case of the true Rat, some carnivorous tendencies—or which feed on the
softer and more nutritious productions of vegetables, as the oily kernels
of nuts, &c.—the crowns of these teeth are not subjected to that amount
of wearing down which requires a prolonged or unlimited growth o*.
dental tissue to replace the lost material. Hence, in some genera, as the
Rat, the Porcupine, no more dental tissue is organised after the crown
is formed than is necessary to fix it firndy in the jaw. These teeth,
therefore, soon acquire roots of ordinary proportions. In a second cate-
gory of the order, we find the molars with short roots tardily developed,
like the same teeth in the Horse and the Elephant. These are they