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100 HUNTER ON THE TEETH.

and thence stands directly opposed to it ; but for this circum-
stance the Grinders would reach farther back in the Upper-Jaw
than in the Lower, which is not commonly the case.

cones, with more complex shallow divisions, and a larger tnberculate
posterior ridge characterise the second true molar. The last molar is of
great antero-posterior extent, owing to the development of the posterior
ridge into a cluster of tubercles. The four primary cones are distin-
guishable on the fore part of the surface of the crown. In the Wart-
hogs (Phacochcerus), the molar series is reduced by the suppression of p. 1
and p. 2. The last true molar in the Wart-hogs is very remarkable for
its extent and complexity. It is, perhaps, the most peculiar and
complex tooth in the Mammalian series. The surface of the crown
presents three series of enamel islands, ranged in the long axis of the
grinding surface ; each row consists of eight or nine islands. Those of
the middle row are elliptic and simple ; those of the inner and outer
rows are sometimes sub-divided into smaller islands. Each island or
lobe consists of an abraded column of dentine encased by thick enamel,
and the whole are united together to form the crown by abundant
cement which fills up the interspaces, and gives an external covering to
the whole tooth. The molar series in the Hippopotamus consists of
The first premolar has a simple corneal crown and a
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single root. It comes early into place at some distance in advance of
the second, and is soon shed. In the existing Hippopotamus, the other
premolars and molars form a continuous series ; but in the Hippopotamus
Major, of the pliocene strata, the second premolar was in advance of the
third by an interval equalling its own breadth. The third and fourth
premolars retain the conical form, but are of larger size, and present
one or two longitudinal grooves on the outer surface, which give the
crown, when worn, a lobed appearance. The true molars are each
divided into two cones or lobes by a wide transverse valley. Each cone
is again sub-divided by a narrow, antero-posterior cleft, into two half
cones, with their flat sides next each other. When moderately worn,
each half cone presents a trefoil of enamel ; but when worn to the base,
the surface of each lobe presents a quadrilobate figure. The crown of
the third molar has a fifth smaller cone developed behind the two
normal pairs of half cones.
In the Paloeotherium, another eocene herbivore, the crown of the
molar is divided into an anterior and posterior part, by an oblique
fissure, continued from near the middle of the inner surface of the crown
obliquely outwards and forwards, two thirds across the tooth. This
fissure enlarges at its termination, so as to mark a division of the
anterior part of the crown into internal and external lobes. The pos-
terior division of the crown is likewise divided into inner and outer
lobes by a short, wide valley or fissure, which extends forward from the