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68 HUNTER OX THE TEETH.
almost perpendicularly, their bodies being turned a little for-
wards. Their fangs are much shorter than those of the

lower jaw, the incisors are long, straight, and sub-compressed : the third
is the smallest, the second rather larger than the first. An excessive
development of the canine and incisor teeth marks the genus Hippopota-
mus. Of the incisors there are four in each jaw; the lower attain the
most remarkable development. The two median lower incisive tusks
are cylindrical, of great size and length, and are worn obliquely on th*
outer and upper side of their extremity ; the deeply-implanted basal
portion is grooved longitudinally : the two outer incisors are likewise
cylindrical and straight, but of smaller size ; they exhibit an abraded
surface towards the inner side of the apex. A large persistent matrix occu-
pies the excavated base of these teeth, and provides for constant growth
and reparation. An ancient form of Hippopotamus which formerly
inhabited India retained the typical number of incisors J; f . .
Amongst Perisso-dactyle Ungulata that number is retained in the
genera Equus and Tapirus. In the Horse, the crowns of the incisors
form the arc of a circle at the extremity of each jaw. They are distin-
guishable from the incisors of all other animals by the vertical fold of
enamel which dips down into the substance of the crown from its broad,
flat upper surface, like the inverted finger of a glove. In the moderately-
worn tooth, the fold of enamel remains surrounded by a cavity, which is
partly filled by cement and partly by the debris of the food. This
constitutes the "mark" of horsedealers. The mark disappears in the aged
animal when the tooth becomes worn still lower than the fold of enamel.
In the different species of Rhinoceros, the incisor teeth present consider-
able variation. They may be absent, and when present they differ
much in form and proportions, and it has been remarked that their
development has a close relation to the development of the nasal
weapon. The two-horned Rhinoceroses of Africa, which have one or
both of the horns largely developed, and a great extinct species (Bh.
tichorinus) in which the horns attained a prodigious size, are instances of
the total absence of these teeth in the adult condition. The Sumatran
two-horned Rhinoceros, in which the horns are but moderately developed,
exhibits incisors in both jaws ; they also exist in both the one-horned
species ; but these teeth attained their largest development in an extinct
hornless Rhinoceros (Rh. incisivus.) The Tapir presents six incisors in
each jaw. The upper median has a broad trenchant crown separated by
a transverse channel, into which the wedge-shaped crown of the Lower
incisor fits, from a basal ridge. The outermost pair of the upper
incisors are large and canine-like, the lower are unusually small.
In the extinct genus Dinotfu rium the incisors are represented by two
large tusks implanted in the deflected extremity of the lower jaw
The crowns of these tusks graduallj decrease to a point ; they are bent