small incisors, in the hare and rabbit, being so placed immediately behind the large
pair, as to receive the appulse o f the sing le pair o f incisors in the lower jaw.
In the Toxodon the position o f the incisors, in the same transverse line, might
lead to the inference, that th ey were opposed by a corresponding number in the
lower j a w ; but the numerous examples o f inequality, in the number o f incisors, in
the upper and lower jaw s o f ex istin g mammalia, forbid any conclusion on this
point.* The sockets o f the small mesial incisors o f the Toxodon (s s, PL I I I .) gradually
diminish in size, as they penetrate the intermaxillary hones, and we may,
therefore, infer that the pulp was gradually absorbed in the progress of their development
; and that, lik e ordinary incisors, their growth was o f limited duration,
and their lodgment in the jaw effected by a sin g le conical fang.
I may observe, that the formation o f a fang is the nece ssary consequence o f
the gradual absorption o f the matrix or pulp o f a tooth ; for the pulp continues, as
it diminishes in size, to deposit ivory upon the inner surface o f the cavity o f the
tooth from which it is receding, and the tooth or fang thus likewise progressively
diminishes in size. Th e formation o f the so cket proceeds uninterruptedly, and
the bone encroaching upon the spac e le ft b y the tooth, c lose ly surrounds the
wasting fang, and affords it a firm su p p o r t; and thus an inference may be drawn
from the form o f the so cket alone, as to whether the tooth it contained had or had
not one or more conical fangs, and consequently whether its growth was temporary
or uninterrupted.
A pply ing this reasoning to the molar teeth o f the Toxodon, we infer that
their growth, lik e those o f most o f the Phytiphagous Rodents, o f the Megathe-
rmm and Armadillo, was perpetual, because their so ckets are continued o f uniform
size from the open to the closed ex tr em ity ; and the molar tooth which is preseiwed
proves the accuracy o f the deduction, inasmuch as its base is excavated b y a
large conical cavity for the lodgment o f the pulp, the continued activity o f which
was the compensation here designed to meet the effects o f attrition on the opposite
or grinding surface o f the tooth.
The molar tooth discovered b y Mr. Darwin in the banks o f the Tercero, not
only belonged to the same spe cie s as the skull under consideration, but to an in dividual
o f the same size ; it fits ex a c tly into the so cket n ex t to the posterior one
o f the right side. The figures subjoined o f this molar tooth (F ig . 3, PI. I.; figs. 2
and 3, P i. IV .) almost preclude the n ece ssity o f a description. The transverse
section of the tooth gives an irregular, unequal sided, prism; the two broadest
S I es o which converge to the anterior angle, which is obtusely rounded. The
rnl1„ +• f wntten before an examination of the fragment of a lower jaw, forming part of Mr. Darwin’s
^ ion 0 ossi emams, had led me to suspect that it was referrible to the genus Toxodon; should this sus-
lower the four uneipiial incisors of the upper jaw are opposed to six equal sized ones in the