line, might lead to the inference, that they were opposed by a
corresponding number in the lower jaw; but the numerous examples of
inequality, in the number of incisors, in the upper and lower jaws of
existing mammalia, forbid any conclusion on this point.[12] The sockets
of the small mesial incisors of the Toxodon (_s s_, Pl. III.) gradually
diminish in size, as they penetrate the intermaxillary bones, and we
may, therefore, infer that the pulp was gradually absorbed in the
progress of their development; and that, like ordinary incisors, their
growth was of limited duration, and their lodgment in the jaw effected
by a single conical fang.
I may observe, that the formation of a fang is the necessary consequence
of the gradual absorption of the matrix or pulp of a tooth; for the pulp
continues, as it diminishes in size, to deposit ivory upon the inner
surface of the cavity of the tooth from which it is receding, and the
tooth or fang thus likewise progressively diminishes in size. The
formation of the socket proceeds uninterruptedly, and the bone
encroaching upon the space left by the tooth, closely surrounds the
wasting fang, and affords it a firm support; and thus an inference may
be drawn from the form of the socket 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.
Applying this reasoning to the molar teeth of the Toxodon, we infer that
their growth, like those of most of the Phytiphagous Rodents, of the
Megatherium and Armadillo, was perpetual, because their sockets are
continued of uniform size from the open to the closed extremity; and the
molar tooth which is preserved proves the accuracy of the deduction,
inasmuch as its base is excavated by a large conical cavity for the
lodgment of the pulp, the continued activity of which was the
compensation here designed to meet the effects of attrition on the
opposite or grinding surface of the tooth.
The molar tooth discovered by Mr. Darwin in the banks of the Tercero,
not only belonged to the same species as the skull under consideration,
but to an individual of the same size; it fits exactly into the socket
next to the posterior one of the right side. The figures subjoined of
this molar tooth (Fig. 3, Pl. I.; figs. 2 and 3, Pl. IV.) almost
preclude the necessity of a description. The transverse section of the
tooth gives an irregular, unequal sided, prism; the two broadest sides
of which converge to the anterior angle, which is obtusely rounded. The
outer surface of the tooth (fig. 2, Pl. IV.) is slightly concave in the
transverse direction, but undulating, from the presence of two slight
convex risings which traverse the tooth lengthwise. The inner surface
presents at its anterior part a slightly concave surface, and
posteriorly two prominent longitudinal convex ridges, separated by a
groove which is flat at the bottom, and from the anterior angle of which
the reflected fold of enamel penetrates the substance of the tooth,
advancing obliquely forwards, rather more than half-way across the body
of the tooth. A longitudinal ridge of bone projects from the internal
side of the socket, and fits into the groove above mentioned, and as a
corresponding ridge exists in all the sockets of the grinders, save the
two anterior small ones, we may infer that the five posterior grinders
on each side, had a similar structure to the tooth above described. The
external layer of enamel is uniformly about half a line in thickness; it
is interrupted for the extent of nearly three lines at the anterior
angle, and for more than double that extent at the posterior part of the
tooth, which is consequently worn down much below the level of the rest
of the grinding surface. Where the ivory is thus unprotected by the
enamel, it has a coat of cæmentum, which also fills up the small
interval at the origin of the reflected fold of enamel. On the grinding
surface of the entire tooth, and on the fractured ends of the mutilated
molars, the component fibres, or tubules, of the ivory, are readily
perceptible by the naked eye, diverging from the line which indicates
the last remains of the cavity of the pulp of the tooth, as it was
progressively obliterated during growth.
Although the complication of the grinding surface by the inflection of
simple or straight folds of enamel is peculiarly characteristic of the
Rodent type, we must regard the number of molar teeth, and their
diminution of size as they advance towards the anterior part of the jaw,
in the Toxodon, as indicative of a deviation from that order, and an
approach to the Pachyderms. The common number of grinders in the upper
jaw of Rodent animals is eight, four on each side. In some genera, as
Lemmus, Mus, Cricetus, there are only three on each side, and in
Hydromys and Aulacodus, only two on each side. In Lepus, however, we
find six on each side of the upper, and five on each side of the lower
jaw. The Toxodon, like the Tapir and Hippopotamus, has seven on each
side of the upper jaw: the first in each of these species being the
smallest. It is worthy of notice, however, that the Capybara which
adheres to the Rodent type in the number of its molars, presents in the
vastly increased size, and additional number of component laminæ of the
posterior grinders, an approximation to the pachydermatous character
just adduced, and the bony palate at the same time presents an expansion
between these molars, offering a resemblance to the Toxodon which I have
not found in any other Rodent besides the Capybara.
The most important deviation from the Rodent structure presented by the
teeth, occurs in the direction of the reflected fold of enamel, and such
a deviation might have been inferred, even in the absence of the teeth,
from the structure of the articular surface, or glenoid cavity for the
reception of the condyle of the lower jaw. As the ridge of enamel runs,
as above described, in a direction approaching that of the longitudinal
axis of the skull, it is obvious that the grinding motions of the lower
jaw should be in a proportionate degree in the transverse direction. The
glenoid cavity, therefore, instead of being a longitudinal groove, and
open behind, as in the true Rodents, is extended transversely, and is
defended behind by a broad descending bony process preventing the
retraction of the jaw, and showing marks of the forcible pressure to
which it was subject.
It is worthy of observation that, in the Wombat,—which exhibits the
Rodent type of dentition, and, like the Toxodon, has remarkably curved
molars, but in an opposite direction,—the condyle of the lower jaw is
also extended transversely, and adapted to an articular surface, which
admits of lateral motion in the trituration of the food. In the outward
span of the zygomatic arches, in which Toxodon deviates from the
Rodentia, we may trace a relation of subordinacy to the above structure
of the grinding teeth and joint of the lower jaw: the widening of the
arches giving to the masseter muscles greater power of drawing the jaw
from side to side. The depth of the zygoma bespeaks the magnitude of
these masticatory muscles, and the included space shews that the
temporal muscles were also developed to a degree, which indicates the
force with which the great incisors at the extremity of the jaws, were
used; probably, like the canines of the Hippopotamus, to divide or tear
up by the roots the aquatic plants, growing on the banks of the streams,
which the Toxodon may have frequented.
In the Rodentia, the zygoma, though sometimes as deep as in the Toxodon,
is generally almost straight, and the space included between it and the
cranium is consequently narrow. The zygoma also is placed more forwards
in all true Rodents, than in the Toxodon; and, instead of abutting
against the posterior alveoli, it terminates opposite the anterior ones.
It thus affords such an attachment to the masseter, that this muscle
extends obliquely backwards to its insertion in the lower jaw, at an
angle which enables it to act with more advantage in drawing forwards
the lower jaw,—a motion for which the joint is expressly adapted. In
many Rodents, also, there is a distinct muscle, or portion of the
masseter, which passes through the ant-orbital foramen, which is on that
account of large size. In examining the cranium of Toxodon, with
reference to this structure, it was found that the ant-orbital foramen
was not larger than might have been expected to give transmission to
nerves requisite for supplying with sensibility the large lips, and
whiskers with which the expanded muzzle of this remarkable quadruped was
probably furnished.
Having thus examined the cranium of the Toxodon in its relation, as a
mechanical instrument, subservient to the function of digestion; we next
proceed to consider the structure and composition of those cavities of
the skull which gave lodgment and protection to the organs of _special_
sense, and endeavour to deduce from their structure conclusions as to
the degree in which the organs were developed, and the circumstances
under which the senses were exercised.
The orbit of Toxodon forms the anterior boundary of the zygomatic area;
it is about as distinctly defined as in the Tapir or Dugong, having its
osseous rim less complete than in the Hippopotamus, yet more developed
than in the Capybara, Coypus, and many other Rodentia, in which the
orbit is scarcely distinguishable in the cranium from the small space
occupied by the origin of the temporal muscle.
The lower boundary of the orbit in Toxodon is formed by an excavation in
the upper and anterior part of the zygoma; the upper boundary by a
strong and rugged overarching process of the frontal bone, the posterior
angle of which (_a_, Pl. III.) descends a little way, but leaves a space
of three inches and a half between it and the opposite angle of the
malar bone below (_b_, Pl. II. and III.), the circumference of the orbit
being completed probably by ligament in the recent subject. The cavity
thus circumscribed is remarkable for the preponderance of the vertical
over the transverse or longitudinal diameter, and indicates great extent
of motion of the eyeball in the vertical direction, such as may be
supposed to be well adapted to the exigencies of an amphibious
quadruped. The orbit of the Capybara, or Water-hog, makes a near
approach to the form just described. In the elevation of the
supra-orbital boundary, and its outward projection in the Toxodon, we
perceive an approximation to the form of the orbit in the Hippopotamus,
but the size of the orbit is relatively larger in the Toxodon, which in
this respect manifests its affinity to the Rodentia.
In that part of the bony structure of the auditory apparatus, which is
visible on the exterior of the cranium, the skull of the Toxodon
presents a character in which it recedes from the Rodentia. In these,
the tympanic portion of the temporal bone is remarkably developed,
forming a large bulla ossea between the glenoid cavity and the occiput;
and it always remains disunited to the other elements of the temporal
bone. In the Toxodon the tympanic bone (_c_, Pl. II.) consists of a
rough compressed vertical osseous plate, wedged in transversely between
the occiput and the posterior part of the glenoid cavity. The internal
extremity of this plate points inwards and forwards, representing the
styloid process; behind this is seen the petrous bone, which forms a
small angular protuberance at the basis cranii, and is less developed
than in the Hippopotamus. Anterior to the petrous bone are the orifices
of the Eustachian tube, and carotid canal; external to it is the great
foramen lacerum, for the jugular vein and nervus vagus; and behind it is
the anterior condyloid foramen. The foramen auditorium externum is only
half an inch in diameter, and gives passage to a long and somewhat
tortuous meatus, which passes inwards and slightly forwards and
downwards; its direction being precisely the same as in the
Hippopotamus; it was accompanied, probably, by as small an external
auricle.
But the indications of the aquatic habits of the Toxodon, which are
presented by the osseous parts relating to the senses of sight and
hearing, are of minor import compared with those afforded by the bony
boundary of the nostrils. This boundary circumscribes a large ovate
aperture, the aspect of whose plane is upwards, and a little forwards,
as in the Herbivorous Cetaceans, and especially the Manatee (_Trichecus
Manatus_, Cuv.) In one part of the bony structure of the nasal cavity
the Toxodon deviates, however, in a marked degree from the Cetaceous
structure; I allude to the frontal sinuses, which are exposed by the
fracture of the upper part of the skull. (They are shewn in Plate III.,
and an asterisk is placed on one of the narrow canals of
intercommunication between the sinuses and the nasal passages.) The
posterior orifice of the nasal cavity is relatively larger and wider
than in the Herbivorous Cetaceans, and differs both in form and aspect
in consequence of the greater extent of the bony palate. The Toxodon
further differs from the Manatee and Dugong, in the firm nature of the
connexion of the bones of the head; and it differs from the Hippopotamus
in the strong attachment of the intermaxillary bones to the maxillaries.
There next remain to be described, as far as the shattered condition of
the skull will permit, the relative position, extent, and connexions of
the principal bones composing it.
The _occipital bone_ exhibits a complete confluence of its basilar,
condyloid, and supra-occipital elements. The basilar portion, in
connexion with the corresponding element of the sphenoid bone, describes
a curve whose convexity is downwards. The condyles are large, extended
in the transverse direction, completely terminal, and a little inclined
downwards below the level of the basilar process. The curve of the
articulating surface describes, in the vertical direction, two-thirds of
a circle, indicating that the head must have possessed considerable
extent of motion upwards and downwards upon the atlas; thus, while the
body of the Toxodon was submerged, the head probably could be raised so
as to form an angle with the neck, and bring the snout to the surface of
the water without the necessity of any corresponding inflection of the
spine. Indeed, in the form and position of the condyles, the Toxodon
more nearly resembles the true Cetacea than any other existing mammalia;
and it is only with these that it can be compared in regard to the
aspect of the plane of the occipital foramen, and of the occipital
region of the skull. This is inclined forwards from the occipital
foramen at such an angle, that on viewing the skull from above, not only
the condyles, but the entire circumference of the occipital foramen are
visible. (See Pl. III.) The upper part of the supra-occipital plate
presents a broad rugous depression, indicative of the insertion of
strong cervical muscles, and probably of a _ligamentum Nuchæ_.[13]
The ex-occipital processes advance forwards for about an inch beyond the
condyles, and then suddenly extend outwards at right angles to the
former line, and terminate in the form of vertically compressed bony
plates; the lower rugged margins of which represent or perform the
office of the mastoid processes (_d_, _d_, Pls. II. and III.). The
breadth of the entire occipital region of the skull (fig. 1, Pl. IV.)
appears to have been, allowing for the fractures, about one-third more
than the height of the same part.
The great development of the _tympanic_ bones in the Rodentia, occasions
the intervention of a considerable space between the occipital bone and
the zygomatic process of the temporal; but in the great Toxodon, in
which the sense of hearing was doubtless inferior to that enjoyed by the
small and timorous Rodents, the tympanic bone is reduced to a thin
plate, which is wedged in between the occiput and glenoid cavity. In
this structure, and the consequent posterior position of the glenoid
cavity, there is a close resemblance between the Toxodon and the
Hippopotamus, Tapir, and Rhinoceros.
The _squamous_ element of the temporal bone (N, Pl. II.) forms a small
proportion of the lateral walls of the cranium, and also enters into the
composition of the lateral and superior parts of the posterior region of
the cranium, where two deep fossæ perforated by large vascular foramina,
indicate the junction of the squamous bones with the supra-occipital
bone. The posterior surface of the skull is thus divided into three
broad and shallow depressions, the two lateral facets being slightly
over-lapped by the middle one, at their junction with it. In this
structure the Toxodon resembles the Hippopotamus, and differs
considerably from the Cetacea, in which the occipital region is rendered
convex by the extraordinary development of the brain within.
The _zygomatic_ process of the temporal bone projects boldly outwards at
its commencement, where it is of great strength, and three-sided; the
glenoid cavity extends transversely across the base or inferior surface
of this part; the lateral surfaces converge to form the ridge or upper
boundary of the zygoma. The depth of the glenoid cavity is increased by
a transverse production of bone both before and behind it: the posterior
process (_g_, Pl. II.) descends the lowest, and affords the requisite
defence against backward dislocation of the lower jaw; the pressure of
the condyle against this process is denoted by a well defined,
transversely-ovate, flattened and smooth surface, as if the bone had
been planed down at that part: the anterior transverse boundary is
convex and smooth, and probably formed part of the articulation for the
lower jaw. The lower facet of the zygoma anterior to the glenoid cavity
gradually contracts in breadth, as it advances forward, and at the
distance of three inches from the articular cavity the zygoma changes
from a prismatic to a laminar form. It is at this point that the
zygomatic suture commences, at the lower margin of the arch; whence it
extends directly forwards for more than half its length, and then bends
upwards at a right angle. The zygomatic suture has a similar course in
the Capybara, and Hippopotamus.
The remainder of the zygoma is formed externally by the _malar_ bone (G
Pl. II.), which in its position is intermediate to the Rodent and
Pachydermatous structures. It is not suspended in the middle of the
zygomatic arch, as in the former order; neither does it extend into the
region of the face so far anterior to the orbit as in the Tapir or
Hippopotamus. The exterior line of the malo-maxillary suture defines the
orbit anteriorly; but from this line the maxillary bone extends
backwards, along the inner side of the malar portion of the zygoma,
until it almost reaches the temporo-malar suture; thus abutting by an
oblique surface against nearly the whole internal facet of the malar
bone, and materially contributing to the general strength of the
zygomatic arch. The malar bone is of considerable vertical extent, and
presents a rugged and thickened inferior margin for the attachment of
the masseter. The upper margin of the malar bone is smoothly rounded,
and presents a regular semicircular excavation, forming the lower
boundary of the orbit. The relative magnitude of the zygomata to the
entire cranium far exceeds in the Toxodon that which exists in the
Hippopotamus or any other known Pachyderm. This arises from the great
vertical development of the malar bone behind the orbit, and the
vertical expansion of the temporal portion of the arch. The oblique
position of the zygoma, descending as it advances forwards, is deserving
of attention, as the Toxodon, in deviating from the Pachyderms in these
respects, makes an evident approach to the herbivorous Cetaceans, as the
Dugong and Manatee: in the latter Cetacean we observe a similar
development of the lower part of the zygomatic process of the malar
bone. It is here, also, that we may perceive an indication of a
resemblance between the Megatherium and Toxodon.
There is no discernible trace of the _lachrymal_ bone (E, Pl. II.)
having extended, as in the Hippopotamus beyond the anterior boundary of
the orbit: the lachrymal foramen is situated rather deep in the orbit,
and the bone itself appears to have been of very small size.
The surface of the supra-orbital process of the _frontal_ bone(C, Pl.
II.) is deserving of attention, as it presents a peculiar ruggedness
which is not found in any other part of the skull; the irregularity
seems, as it were, to have been produced by the impression of numerous
small tortuous and anastomosing vessels. In the skull of a Sumatran
two-horned Rhinoceros, in the Museum of the College of Surgeons (No.
816), the circumference of that part of the surface of the skull which
supported the posterior horn, and which includes precisely the same part
of the os frontis, presents the same character, the surface being broken
by numerous vascular impressions. On the supposition that this character
of the supra-orbitary arch in the Toxodon might indicate the
superincumbency of a bony case, I examined the skulls of two Armadillos,
_Dasypus Peba_ and _Das. 6–cinctus_, and found that in the Dasypus
6–cinctus, the supra-orbital ridges, which are slightly elevated, to
support the cephalic plate, presented, in a minor degree, a
corresponding rugosity. May we venture then to conjecture that the
Toxodon was defended by an ossified integument like the Armadillo, or
that it was armed with an epidermic production, analogous to the horn of
the Rhinoceros; or had the rugous surface in question as little relation
with the parts that covered it as the sculptured surface of the malar
bones in the Cavy?
After forming the rugged and prominent supra-orbital processes already
described, the frontal bone continues to send backwards a slightly
elevated ridge or _crista_, circumscribing the origin of the temporal
muscles, but the extent of this ridge, and the disposition of the
inter-orbital portion of the frontal bones cannot be determined in the
present mutilated specimen. The fractures it has sustained are not,
however, wholly unattended with advantage; they expose the structure of
the diploë, which from its coarseness of texture and thickness,
resembles that of the Cetaceous crania; and what is of still more
importance, they also demonstrate the existence and form of the frontal
sinuses.
The cavity of the nose is extensive, and the remains of the ossa
spongiosa superiora testify that the Toxodon enjoyed the sense of smell
to a degree equal at least to that of the Hippopotamus.
The _sphenoid bone_ resembles that of the Hippopotamus, but it
contributes a larger share to the formation of the internal pterygoid
processes (_p_, Pl. II.); these are of a simple form, and more developed
than in the _Hippopotamus_; they project outwards to a greater extent,
and terminate in a point. The sphenoid also sends off a short and thick
pointed process from the posterior part of the base of the internal
pterygoid processes. The ala of the sphenoid does not rise so far into
the orbit, nor does it articulate with the parietal bone, as in the
Hippopotamus; but in this part of its structure, is the same as in the
Rhinoceros. The spheno-palatine foramen is relatively larger than in the
above-named Pachyderms, and is bounded above by the descending orbital
plate of the frontal bone.
The palatal processes of the _palatine_ bones terminate anteriorly
between the last molars, and extend backwards for some distance beyond
the alveolar processes, increasing the extent of the bony roof of the
mouth posteriorly: this is a structure in which the Toxodon deviates
both from the Rodents, and Pachyderms, and resembles the Armadillos
among the Edentata; excepting that the postdental part of the bony
palate in the Toxodon is suddenly contracted in breadth. The
palato-maxillary suture is in the form of a chevron, with the angle
directed forwards, as in the Hippopotamus and Capybara, but truncated.