as this jaw p resents; for the rohust incisors may here he regarded as repre-
r e c h L “ o f “ d "'ith a change in tL ir
In the second moUr, we find an increase in the antero-posterior diameter, and
n the length o f the tooth, and the enamel at the middle o f the outer side makes
a fold which penetrates a h ttle way into the tooth ; the line o f enamel on the
inner side, is slig litly concave and unbroken.
The th ird molar presents an increase o f dimensions in the same directions as
the second ; the enamel on the outer side o f the tooth presents a similar fold, but
It is directed a little more backwards.
In th e fo u rth molar besides a further increase o f size, and a corresponding
hut deeper fold o f enamel m the external side o f the tooth, we have the grinding
surface rendered more complicated by two folds o f enamel entering the substanc!
o f the toolJi from the inner s id e : these folds divide the antero-posterior extent
w a rd ! wards, h°aTlff- w"a y across the substance o f the ivory. obliquely fornnl
- structure as the fourth, which it exceeds
only slightly in size.
mrern*'''' g?oator increase o f size in the
an ero-postenor diameter, which measures two in ch es; but the lateral diameter
IS but sligh tly augmented ; its structure resembles that o f the fifth.
A s these grinding teeth by no means increase in the lateral diameter in the
same proportion as in their antero-posterior diameter, the posterior ones present
u III a greater degree, the compressed form which characterizes the grinders o f
the u pp er jaw o f the Toxodon. naers ot
I t will he seen, however, that there is a difference in the structure o f the
grin ers in this fragment o f the lower jaw and those o f the upper jaw o f the Toxodon.
In the lower grinders there are two folds o f enamel proceeding from the
o n r fo l 1 I '"1" ‘"“ Ist in the upper grinders there is only
Z m e from the inner side; in the lower grinders there is also a fold o f
namel reflected into the substance o f the tooth from the outer surface, while in
tooth merely bent inwards, so as to describe, in the transverse section, a gently
4 i S w ® ’ grindingsurface o f the six th molar, ^ g h fs id e !
Bu t this difference o f structure is by no means incompatible with the co
existenc e o f the two series o f teeth in the same animal, sin ce we find the grinders
o f the upper and lower jaw s presenting differences o f structure o f equal degree in
shall find not only an equal amount o f difference in the structure o f the upper
and lower grinders, but that they deviate from one another in a very similar manner
to that above described in the Toxodon. In this comparison attention should be
confined to the course o f the external enveloping layer o f enamel, leaving out o f
consideration the central crescentic islands o f enamel which constitute the additional
complexity o f the Horse’s grinder. Viewing then the course o f the external
coat o f enamel on the worn surface o f the tooth, we find it describing on th e outer
sid e o f the tooth in the upper jaw an undulating lin e ,—a middle convexity being
situated between two concavities ; on the inner side o f the tooth one fold o f enamel
penetrates to the middle o f the tooth, and on each side o f this there is a smaller
fold. B u t in the lower jaw the line o f enamel on the outer side o f the tooth,
instead o f merely bending outwards midway in its course, is reflected a little
way inwards ; while on the opposite, or inner side o f the tooth, the enamel sends
two extensive folds into the substance o f the tooth, opposite to the interspace o f
which the shorter fold projects from the outer side. Now, on the supposition that
the fragment o f thelower jaw here described belongs to th eTo x od on,th e k ind and degree
o f difference in the complexity o f the grinding surface o f the teeth in the upper
and lower jaw, are remarkably analogous to those which ex ist in the Horse. J have
only further to remark that in the Horse the inflected folds o f enamel, instead o f
being simple and straight with the two constitutive layers in apposition, as in the
Toxodon, are irregular in their course, with cæmentum intervening between the
constitutive layers, which also diverge from each other at their angle o f reflection,
so as to augment the amount o f d ense material which enters into the composition
o f the tooth.
Many analogous examples will readily occur to the experienced comparative
anatomist. T h e Horse has been adduced as one to which reference can very readily
be made; but I would also cite the Sumatran Rhinoceros, the skull o f which, in
the Hunterian collection, has already been alluded to. In this spe cie s the anterior
grinders, in both jaws, are small and simple, and increase in complexity as
they recede backwards. T he third superior grinder (fig. 8, PL V .) presents a
sing le fold o f enamel, reflected obliquely forwards from the inner side half-way
across the tooth ; the outer line o f enamel describes a simply undulating line. The
opposite grinder o f the lower jaw (fig. 9, PI. V .) has only one-half the breadth o f
the upper one, but has its grinding surface further complicated by having two in flected
folds o f enamel from the inner side, and one shorter and broader fold from
the outer side. This tooth, therefore, presents a close resemblance to one of the
posterior grinders o f the lower jaw o f the Toxodon, but differs essentially in being
o f limited growth, and consequently in having fangs.*
* Besides the relation to food requiring much comminution, which teeth with persistent pulps bear, they
e also connected with the longevity of the individual. The term of life in a herbivorous animal, with grinders