transverse plates were thinner and more numerous ; they are also
deeper, and more subdivided into digitated summits : and we may
infer from the resulting increase of the dense enamel which enters
into the formation of the broad grinding surface, that the extinct
species had to subsist on a coarser and more ligneous vegetable
diet, in conformity with the colder regions of the ancient world
in which its peculiar warm clothing of wool and hair adapted it
to exist.
The phenomena of the course and changes of the dentition
of the Elephas Africanus have a close analogy with those of the
preceding species with more complex teeth. The first molar has
a four-ridged crown, with two long diverging fangs; the specific
characters are least manifested in this early-developed tooth : they
become more marked in the subsequent molars. The second has
seven plates, five of normal size and already manifesting the characteristic
median expansion; the first and last plates are much
smaller: the length of this tooth is two inches and a quarter.
The third molar manifests an increase in the size and thickness
of the coronal plates, and of the width of their cement-filled
interspaces, hut not any in the number of the divisions; the first,
however, presents more of the normal proportions. The fourth molar
manifests a marked increase of size and especially of breadth, but its
crown is not divided into more than seven plates. The fifth molar,
(PI. 148, fig. 4) which is about seven inches in length, has eight
or nine coronal plates. The sixth molar, eight or nine inches in
length has from ten to twelve plates in the lower jaw.
The molar teeth in all the species of Elephant succeed each
other from behind forwards, moving, not in a right line, but in the arc
of a circle (PI. 146, fig. 1): the position of the growing tooth in the
closed alveolus (m 5) is almost at right angles with that of the molar in
use, the grinding surface being at first directed backwards in the upper
jaw, and forwards in the lower jaw, and brought by the revolving
course into a horizontal line in both jaws, so that they oppose each
other, when developed for use. The imaginary pivot on which
the grinders turn is next their root in the upper jaw, and is
next the grinding surface in the lower jaw ; in both towards the
frontal region of the skull: viewing the upper and lower molars
as one complex whole, subject to the same revolving movement,
the section dividing such whole into upper and lower portions
runs parallel to the curve described by that movement, the upper
being the central portion or that nearest the pivot, the lower the
peripheral portion: the grinding surface of the upper molars is
consequently convex from behind forwards, and that of the lower
molars concave: the upper molars are always broader than the
lower ones. The bony plate (ib. fig. 2, a) forming the sockets oi
the growing teeth is more than usually distinct from the body ol
the maxillary, and participates in this revolving course, advancing
forwards with the teeth. The partition between the tooth in use
and its successor is perforated near the middle ; and in its progress
forwards that part next the grinding surface is first absorbed;
the rest disappearing with the absorption of the roots of the preceding
grinder.
There are few examples of organs that manifest more strikingly
the adaptation of a highly complex and beautiful structure to
the exigencies of the animal endowed with it, than the grinding
teeth of the Elephant. We perceive, for example, that the jaw is
not encumbered with the whole weight of the massive tooth at
once, but that it is formed by degrees as it is required; the subdivision
of the crown into a number of successive plates, and of
these into subcylindrical processes, presenting the conditions most
favourable to progressive formation. But a more important advantage
is gained by this subdivision of the tooth: each part is formed
like a perfect simple tooth, having a body of dentine, a coat of enamel,
and an outer investment of cement: a single digital process may
be compared to the simple canine of a Carnivore ; a transverse
row of these, therefore, when the work of mastication has commenced,
presents, by virtue of the different densities of their
constituent substances, a series of cylindrical ridges of enamel,
with as many depressions of dentine, and deeper external valleys
of cement: the more advanced and more abraded part of the
crown is traversed by the transverse ridges of the enamel inclosing
the. depressed tracts of the dentine, and separated by the