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62 HUNTER ON THE TEETH.
larly ; and the posterior is concave and sloping, so that the
cutting edge is almost directly over the anterior surface.
These surfaces are broadest and the Tooth is thinnest at the
cutting edge, or end of the Tooth, and thence they become
gradually narrower and the Tooth thicker towards the neck
7
which are displaced vertically and succeeded by permanent tusks. It is,
however, in the male sex and in the upper jaw only that these teeth
project from the gum ; in the female Dugong both upper and lower
incisors remain concealed throughout
life. The superior incisors are
two in number in both sexes ; in the male the extremity only of the
tusk projects from the jaw, at least seven-eighths of its extent being
lodged in the socket. The male tusk is subtrihedral, moderately and
equally curved, and its extremity is bevelled off to a cutting edge like
the scalpriform incisors of the Rodents. In the young American
Manatee (Manatus Americanus) each premaxillary bone supports a deci-
duous tusk, which, however, is not replaced
In the gum which covers
the deflected portion of the ramus of the lower jaw in the new-born
Manatee, six depressions for rudhnental teeth are to be observed; in one
of these an incisor tooth was observed by Stannius.
Incisors are present in all the species of the order Marsupialia, but
they vary in number, in some instances exceeding that of the Mammalian
type. In the carnivorous Dog-headed Thylacine and Ursine Dasyure,
there are eight incisors in the upper jaw, and six in the lower ; in the
Dasyures these teeth are simple in structure, and are arranged in
a regular semicircle. The existing Australian genus Myrmecobius, and
the extinct Marsupial genera Phascolotherium and Amphitherium, found in
the oolitic slate at Stonesfield in Oxfordshire, afford examples in which
the incisors are separated from each other and from the canines by
vacant spaces. Ten incisor teeth in the upper jaw and eight in the
lower are found in the Opossums (Didelphys) ; the two middle upper
incisors are longer than the others, from which they are separated by
a short interspace. In the Tapoa {Phalangista fuliginosa), the Koala
{Phascolarctos cinereus), the Kangaroo Rats {Hypsiprymnus), and the Kan-
garoos (Macropus), there are six incisors in the upper jaw, whilst the
lower are reduced to two. The two anterior upper incisors are more
than twice as large as the lateral in the Koala; the same teeth also
attain large dimensions as compared with the others of the series in the
Kangaroo Rats : in the latter genus their pulps are persistent. The feeble
development of the lateral superior incisors in Phascolarctos and Hypsi-
prymnus is succeeded by their total suppression in the Marsupial
Rodent, the Wombat. In the Wombat (Phascolomys), the incisors are
two in the upper and two in the lower jaw ; they are genuine dentes
scalprarii with persistent pulps, although inferior, especially in the
lower jaw, in length and curvature to those of the true Rodents