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86 HUNTER ON THE TEETH.
often bifurcated. There are two cavities in each fang, one
towards each edge, leading to the general cavity in the body of
the Tooth. These two cavities are formed by the meeting of

somewhat recurved, and the base obscurely notched before and
behind. The extinct Phascolotherium had three premolars and four
molars in the lower jaw ; the first premolar and fourth molar
are the smallest. The five larger teeth are each tricuspid, the middle
cusp being the largest ; but the crown is girt by a basal ridge, which
projects a little beyond the anterior and posterior cusps, and gives the
tooth a quinque-cuspid character. The dentition of the genus Amphi-
therium resembles, in many respects, that just described ; but not only
are the molars increased in number, as in Myrmecobius, but the pre-
molars are also six on each side in the lower jaw. The dentition of the
upper jaw in Phascolotherium and Amphitherium is at present unknown,
but it probably corresponded with that of the lower. In the Opossum
(Didelphys), the formula of the molar series is p. § ' %] m,.\\ \ \ . The molars
are tuberculous in structure, and depart more or less from the type of the
true flesh-feeders. In Phascolarctos, Hypsiprymnus, Macropus, and Phas-
colomys, the premolars are reduced to \\ \\ , whilst the molars are %\ £] .
In the Koala (Phascolarctos cinereus) the premolar is compressed, and has
a cutting edge ; the molars are quadricuspid, with the rudiment of a
" cingulum." In the Potoroos, or Kangaroo Rats (Hypsiprymnus), the
single premolar is of large size, and has a peculiar trenchant form : in
the young tooth, the crown is indented, especially on the outer side, by
a series of vertical grooves. In the Arboreal Potoroos of New Guinea,
the premolar nearly equals in fore and aft extent, the three succeeding
molars. The true molars are surmounted by four trihedral, pyramidal
cusps ; the internal angles of opposite cusps are continued into each
other across the grinding surface, giving rise to two transverse ridges.
In the old tooth, the cusps and ridges become worn down. In the Kan-
garoos (Macropus), the crowns of the true molars present two transverse
ridges, with a broad anterior talon, and a narrow hinder one. In most
species, a spur connects the anterior with the posterior ridge, and
another the anterior ridge with the front talon. The molars in the
gigantic, extinct Diprotodon were five in number on each side in both
jaws ; they presented the same double, transversely-ridged conformation,
with the addition of an anterior and posterior low basal ridge. The
dentition in the Wombat (Phascolomys) is peculiar amongst Marsupialia.
The premolar has lost the compressed character it has in the Koala and
Kangaroos. It presents a wide, oval, transverse section : the superior
premolar tooth is traversed on the inner side by a longitudinal groove.
The superior molars are each also divided by a wide, internal, longitu-
dinal groove, into two prismatic lobes, one angle of each prism being
directed inwards. The inferior molars arc, in like manner, divided into