Page 81 - My FlipBook

OF THE [NCISORES. 65
narrower from its cutting edge to tlie extremity of its fang.
But in a side-view it is thickest or broadest at its neck, and
thence becomes gradually more narrow, both to its cutting edge,
and to the point of its fang.

On turning to the order Quadrumana, we find a genus (Cheiromys) in
which, as in the Marsupial Wombat, the Suctorial Bats, and the Insecti-
vorous Shrews, the dentition is modified in analogy with the Rodent
Type. In the Aye-aye (Cheiromys Madagascariensis) canines are want-
ing, and a wide interval separates two large curved scalpriform incisors
from the molar series. These teeth differ from the incisors of the
Rodents in having an entire investment of enamel, which is, however,
thickest on the anterior surface. In the slow Lemurs (Stenops), and in
the true Lemurs, or Makis, there are four incisor teeth in each jaw. In
the Stenops tardigradus the first upper incisor is larger than the second :
in the true Lemurs the upper incisors are small and vertical, with
expanded crowns ; an interval separating the two on the right side from
those on the left. The inferior incisors with the canine are long, narrow,
compressed, and procumbent. In the Platyrhine Quadrumanes of
America, as in the Quadrumana of the Old World, the incisors are f : f
:
;
the Sakis manifest the lemurine character of long narrow inferior incisors.
In Cebus the inferior incisive teeth have broad, thick, wedge-shaped
incisive crowns; and this character is generally retained throughout the
Quadrumanous series. The Simice of the Old World present the nearest
approach to human conformation offered by the animal kingdom. The
large black tailless Apes of Africa (Troglodytes), and the red Orangs
of the Indian Archipelago (Pithecus), afford the closest approximation.
It will be interesting here to compare this part of the dental series in
the two best-known African species—the Gorilla and Chimpanzee, and
in the two species of Orang, with the human permanent incisors.
The characteristics of the human incisors are their smallness of size
in proportion to the other teeth and to the entire skull ; their near
equality of size with each other ; their thin wedge-like form, and their
vertical, or nearly vertical, implantation. In the upper jaw the two
middle incisors slightly exceed the lateral in breadth ; the posterior
surfaces of both are smooth and slightly concave. The middle incisor
presents a slightly-developed basal ridge on its posterior surface; but sucli
elevation is entirely absent on the posterior surface of the lateral one.
The external angle of the crown of the lateral incisor is not rounded
off, and is in contact with the canine. In the lower jaw the incisors
are somewhat smaller than in the upper, and the breadth of the central
is slightly inferior to that of the lateral; the outer angle of the crown of
the latter is entire. In the Gorilla (see plate I) and Chimpanzee the inoism
teeth are not only absolutely .much larger (thicker ui hi broader) than in
man, but they are of i'ar greater size in proportion to the molar series