and with a change in their direction. The second molar presents an
increase in antero-posterior diameter and in length, and the enamel
of the middle of the outer side makes a fold which penetrates a little
way into the tooth; the cement covering the inner side is slightly
concave and unbroken. The third molar presents an increase of
dimensions in the same directions as the second; the enamel on the
outer side of the tooth presents a similar fold. In the fourth or first
molar, besides a further increase of size and a corresponding but
deeper fold of enamel on the external side and nearer the anterior
part of the tooth, the grinding surface is rendered more complicated
by the two folds of enamel entering the substance of the tooth
from the distinct plate on the middle of the inner side: these folds
divide the antero-posterior extent of the tooth into three nearly equal
parts; they are both directed obliquely forwards, the hinder one
goes half-way across the. substance of the dentine. The fifth molar
presents the same structure as the fourth, which it exceeds only
slightly in size. The sixth molar has a much longer antero-posterior
diameter, which measures two inches ; hut the lateral diameter is
but slightly augmented ; its structure resembles that of the fifth. The
outer coat of enamel extends over half the anterior and posterior
ends of the tooth.
The partial disposition of the enamel upon the molars of the
Toxodon is peculiar to that genus ; hut the enamel is continued, as
in other rootless teeth, to the open end of the implanted base; it is
thinner than in the Rhinoceros. The unenamelled parts of the tooth
are coated by a thin layer of cement. The entire body of the tooth is
composed of compact dentine, the pulp-fissure which penetrates the
middle of the lobes defined by the inflected folds of the enamel, extends
from the apex of the open basal pulp-cavity to the grinding surface.
The dentinal tubes are J*th of a line at their origin, and radiate
in directions vertical to the superficies of the tooth, and of the
inflected enamel-folds, and are but little inclined upwards from the
horizontal plane. They maintain their original diameter, and their
relative distance from each other, viz. ^,th of a line, to near their
peripheral ends. The dentinal cells are sub-hexagonal, and about
» th of an inch in diameter in the peripheral part of the substance.
In the discontinuity of the enamel covering the molars, the Toxodon
differs from all known Pachyderms, and manifests a slight approach
to the Bruta.
214. Elasmotherium{Y).—This name has been given to an extinct
Pachyderm with rootless molars, surpassing the Toxodon in size, and
of which only the lower jaw and its dentition are as yet known ; but
the characters of the teeth are sufficiently remarkable to call for
notice here. The molar teeth of the Elasmotherium are five in
number in each ramus of the jaw, the anterior one being very
small; the penultimate one is the largest, measuring three inches
in the antero-posterior diameter, and two inches in the transverse
diameter of the crown. The enamel is remarkable for its beautiful
undulating folds; but its general disposition most resembles that
in the inferior molars of the Rhinoceros (2). The teeth of the
Elasmotherium differ from those of the Rhinoceros, and resemble
those of the Horse in the great depth to which they are implanted in
the jaw, before being divided into roots: the socket of the penultimate
grinder extends, in fact, to the lower margin of the jaw
without any indication of partitions for the lodgment of fangs:
there is no trace of incisive teeth in the portion of symphysis which
is preserved, and which extends a little more than three inches in
advance of the first small molar. The above account is taken from a
cast and the description by Cuvier, in the ‘ Ossemens Fossiles’, 4to.
tom. ii. pt. i, p. 96. The original is preserved in the Museum of
Moscow, and is unique ; it was discovered in the frozen drift or
diluvium of Siberia.
215. Rhinocerotidce.—The present family of anisodactyle Pachyderms
includes the typical Rhinoceros, the extinct Acerotherium which
had no horn, and the equally hornless small existing genus Hyrax.
The essential characteristics of the dentition of the genus Rhinoceros
are to be found in the form and structure of the molar teeth. In the
first place, they differ essentially from those of the Horse or Elasmothere
by being implanted by distinct roots. In the upper jaw the crown is
(1) ’i\a<Tfia a plate, dijptov beast : in allusion to the plicated plates, of enamel in the
substance of the molar teeth.
(2) Compare figure 12 in Plate 136, with p. 3, fig. 11 in Plate 138.