by a long undivided continuation of the crown ; when the force begins
to be exhausted the matrix is simplified by the suppression of the
enamel organ, and the dentinal pulp continues to be reproduced only at
certain points of the base of the crown, which, by their elongation,
constitute the fangs. The Beaver and other Rodents in the second
category of the order, according to the implantation of the molar teeth,
exemplify the above condition ; but in the Capybara, Dolichotis, (PI.
104, figs. 2 & 3), and other Rodents with rootless molars, the reproduction
of the molar like that of the incisor teeth, appears to
continue throughout the animal’s existence. The rootless and perpetually
growing molars are always more or less curved; they derive
from this form the same advantage as the incisors, in the relief of
the delicate tissues of the active vascular matrix from the effects
of the pressure which would otherwise have been transmitted more
directly from the grinding surface.
The complexity of the structure of the crown of the molar teeth,
and the quantity of enamel and cement interblended with the dentine,
are greatest in the rootless molars of the strictly herbivorous Rodents.
The crowns of the rooted molars of the omnivorous Rats and
Mice are almost as simple as the tuberculate molars of the Bear
or of the Human Subject, which they appear to typify. They
are at first tuberculate, as shown in PI. 105, fig. 9 ; when the
summits of the tubercles are worn off, the inequality of the grinding
surface is for a time maintained by the deeper transverse folds of
enamel, the margins of which are separated by alternate valleys
of dentine and cement, as shown in PI. 105, fig. 10, 3 a; but
these folds sinking only to a slight depth are in time obliterated,
and the grinding surface is reduced to a smooth field of dentine
with a simple border of enamel, as shown in the upper tooth of
figure 10, 3. A similar change in the grinding surface, consequent
on age and use, is shown in the molars of the Souslik, or Ground
Squirrel, PI. 105, fig. 3, b, and in those of the Gerbille, fig. 8, and
is common to all that possess roots.
Examples of various forms assumed by the inflected folds
of enamel in the molars of the Rodentia are given from the works
of the Cuviers in PI. 105. It will be seen that these folds have
a general tendency to a transverse direction across the crown of the
tooth. Baron Cuvier has pointed out the concomitant modification
of the shape of the joint of the lower jaw, which almost restricts
it to horizontal movements, to and fro, in the direction of the
axis of the head, during the act of mastication. When the folds
of enamel dip in vertically from the summit to a greater or less
depth into the substance of the crown of the tooth, as in those
molars which have roots, the configuration of the grinding surface
varies with the degree of abrasion, of which examples have already
been cited; but in the rootless molars where the folds of enamel
extend inwards from the entire length of the sides of the tooth, the
characteristic configuration of the grinding surface is maintained
without variation, as in the Guinea-pig, (fig. 16), the Capybara,
(fig. 17), and the Patagonian Cavy, (PI. 104, figs. 2 & 3).
The whole exterior of the molar teeth of the Rodentia is
covered by cement, and the external interspaces of the enamel-
folds are filled with the same substance. In the Chinchillidte and
the Capybara where the folds of enamel extend quite across the
body of the tooth, and insulate as many plates of dentine, these
detached portions are held together by the cement; such folds of
enamel are usually parallel, as in the large posterior lower molar
of the Capybara, which in shape and structure offers a very close
and interesting resemblance to the molars of the Asiatic Elephant.
The partial folds and islands of enamel in the molars of the
Porcupine (fig. 13), and Agouti, (fig. 14), typify the structure of
the teeth of the Rhinoceros : the opposite lateral inflections of enamel
in the molars of the Gerbille, (fig. 8), and Cape Mole-rat, (fig. 11),
represent the structure of the molars of the Hippopotamus : the
double crescentic folds in the Jerboa (fig. 7), sketch out, as it were,
the characteristic structure of the molars of the Anoplothere and
Ruminantia.
Although, as has been shown, the molar teeth in many Rodents
are rootless and of unlimited growth as in the Edentata, in none
is enamel absent or vascular dentine, as the chief constituent of
the tooth, present: these essential differences characterise the molars
of those Rodents which by use have their grinding surface reduced
d d 2