the malar, as above described: they ascend and join the frontal and
nasal bones: their outer surface is almost vertical, smooth, and
slightly undulating; perforated at its posterior part by the ant-orbital
foramen, and joined anteriorly to the intermaxillaries by a suture
running in the sigmoid direction (as shewn in Pl. II.) from the middle
of the nasal cavity, to within four inches of the anterior boundary of
the upper jaw. We have, in the position and extent of this suture, and
the absence of tusks and their large prominent sockets, a most important
difference between the Toxodon and Hippopotamus. The chief peculiarity
in the maxillary bones, obtains in the arched form of the alveolar
processes, corresponding to the shape and position of the grinders above
described, and which are peculiar among known mammalia to the present
genus. The palatal surface of the maxillary bones is obliquely
perforated by two large foramina, from which two deep longitudinal
grooves extend forwards, and are gradually lost; we find the posterior
palatine foramina represented by similar grooves and foramina in the
Capybara.
The _intermaxillary_ bones (D, Pls. II. and III.), though large, are
relatively of less extent than in the Rodents generally. The nasal
processes do not reach the frontal bone, but are limited to the anterior
half of the nasal boundary; approaching in this respect to the
Herbivorous Cetacea. In the outward expansion of their anterior
extremities, the intermaxillaries resemble those of the Hippopotamus, in
which, however, this character is more strongly marked. The
intermaxillaries in the Hippopotamus are also much less firmly united to
the maxillary bones than in the Toxodon, and are consequently commonly
lost in the fossil crania. On the palatal surface of the intermaxillary
bones there are two grooves which diverge forwards from the line of the
suture; and anteriorly to these grooves there are the two large anterior
palatine foramina. The maxillo-intermaxillary sutures on the palate
converge as they extend backwards to a point; there appears to have been
a fissure left between this suture and the mesial suture of the
intermaxillaries; in which structure the Toxodon resembles the
Hippopotamus.
After summing up the different affinities, or indications of affinity,
which are deducible from the cranium of this most curious and
interesting fossil mammal, we are led to the conclusion, assuming it to
have had extremities cased in hoofs, that it is referrible to the Order
Pachydermata. But the structure, form, and kind of teeth in the upper
jaw, prove, indisputably, that the gigantic Toxodon was intimately
related to the Rodent Order. From the characters of this order, as
afforded by the existing species, the Toxodon, however, differs in the
relative position of the supernumerary incisors, and in the number, and
direction of the curvature, of the molars. If, moreover, the lower jaw,
next to be described, belong, as I believe, to the Toxodon, the dental
character of the genus will be _incisors_ ⁴⁄₆; _pro laniariis diastema_;
_molares_ ⁷⁄₇ ⁷⁄₇.
The Toxodon again deviates from the true Rodentia, and resembles the
Wombat, and the Pachyderms, in the transverse direction of the articular
cavity of the lower jaw.
It deviates from the Rodentia, and resembles the Pachydermata in the
relative position of the glenoid cavities and zygomatic arches, and in
many minor details already alluded to.
In the aspect of the plane of the occipital foramen, and occipital
region of the skull; in the form and position of the occipital condyles;
in the aspect of the plane of the anterior bony aperture of the
nostrils; and in the thickness and texture of the osseous parietes of
the skull, the Toxodon deviates both from the Rodentia and existing
Pachydermata, and manifests an affinity to the Dinotherium and Cetaceous
Order, especially the Herbivorous section.
At present we possess no evidence to determine whether the extremities
of the Toxodon were organized on the ungulate or unguiculate type, nor
can we be positive, from the characters which the skull affords, that
the genus may not be referrible to the _Mutica_ of Linnæus;[14] although
the development of the nasal cavity and the presence of large frontal
sinuses render it extremely improbable that the habits of this species
were so strictly aquatic, as the total absence of hinder extremities
would occasion.
Where the dentition of a mammiferous animal is strictly carnivorous,
this structure is obviously incompatible with a foot incased in a
hoof:—but where the teeth are adapted for triturating vegetable
substances the case is different. If animals so characterized are of
small size and seek their food in trees, or if they burrow for roots or
for shelter, the vegetable type of dentition must co-exist with
unguiculate extremities, as in the Edentata and Rodentia generally: but
the largest genus (Hydrochærus) of the Rodent Order, whose affinity to
the Pachydermata is manifested in its heavy shapeless trunk, thinly
scattered bristly hair, and many other particulars, has each of its toes
inclosed in a miniature hoof.
The affinity above alluded to, is too obvious to have escaped popular
notice, and the Capybara, from its aquatic habits, has obtained the name
of Water-hog. It is highly interesting to find that the continent to
which this existing aberrant form of Rodent is peculiar, should be found
to contain the remains of an extinct genus, characterized by a dentition
which closely resembles the Rodent type, but manifesting it on a
gigantic scale, and tending to complete the chain of affinities which
links the Pachydermatous with the Rodent and Cetaceous Orders.
ADMEASUREMENTS OF THE CRANIUM OF TOXODON. feet inches lines
Extreme length 2 4
Extreme breadth 1 4
Extreme height, (exclusive of the lower jaw) 10
Length of zygomatic process 1 1 6
Depth or vertical extent of do. 6
Transverse extent of zygomatic fossa 6
Transverse diameter of cranium between the zygomatic arches 5
Transverse diameter of occipital plane of the cranium 1
From the outside of one condyle to that of the opposite condyle 8 6
Length of the bony palate 1 6
Extreme breadth of ditto 6
Breadth of palate at the intermaxillary suture 2 6
Breadth of palate behind the molar alveoli 3
Longitudinal extent of the molar alveoli 9 6
Longitudinal extent of the diastema 5 6
Transverse diameter of posterior nasal aperture 3 9
Transverse diameter of occipital foramen 3
Transverse diameter of glenoid cavity 4 6
Antero-posterior do. of ditto 1
DESCRIPTION OF FRAGMENTS OF A LOWER JAW AND TEETH OF A TOXODON.
Found at Bahia Blanca, in latitude 39° on the East coast of South
America.
In looking over some fragments of jaws and teeth, forming part of Mr.
Darwin’s collection of South American mammiferous remains, and which had
been set aside with mutilated specimens referrible to species belonging
to the family of Edentata, my attention was caught by the appearance of
roots of teeth projecting, in a different direction from the grinders,
from the fractured anterior extremity of a lower jaw, and I was induced
to examine minutely the structure of the teeth in this specimen, and to
search the collection for corresponding fragments. The result was the
discovery of portions of the two rami, and the commencement of the
symphysis of a lower jaw, containing anteriorly the roots of six
incisors, and at least six molars on each side; but as the rami had been
fractured through the middle of the sixth alveolus, the number of
grinders may have corresponded with those in the upper jaw of the
Toxodon.
The most perfect of these fragments is figured in Pl. V. figures 1 and
4; figure 2 shows the form of the teeth in transverse section, and the
disposition of the enamel upon the grinding surface of the molars on the
right side, as restored from a comparison of the fractured teeth in the
two rami. From the remains of the symphysis shown at fig. 4, it will be
seen that the jaw was remarkably compressed, or narrow from side to
side; while the rami (fig. 1.) were of considerable depth, in order to
give lodgment to the matrices and bases of grinders enjoying
uninterrupted growth.
The pulps of the six incisors in this lower jaw are arranged in a pretty
regular semicircle, whose convexity is downwards; the teeth themselves
are directed forwards, and curved upwards, like the inferior incisors of
the Rodentia. The form and degree of the curvature are shown in the
almost perfect incisor (Pl. V. fig. 5) which corresponds with the left
inferior incisor of the lower jaw, and was found in the same stratum,
but belonged to another individual.
These incisors are nearly equal in size: they are all hollow at their
base, and the indurated mineral substance impacted in their basal
cavities well exhibits the form of the vascular pulps which formerly
occupied them. Sufficient of the tooth itself remains in four of the
sockets to show that these incisors, like the nearly perfect one (fig.
5), had only a partial investment of enamel; but though in this respect,
as well as in their curvature and perpetual growth, they resemble the
dentes scalprarii of the Rodentia, they differ in having a prismatic
figure, like the inferior incisors of the Sumatran Rhinoceros, or the
tusks of the Boar. Two of the sides, viz., those forming the anterior
convex and mesial surfaces of the incisor have a coating of enamel,
about half a line in thickness, which terminates at the angles between
these and the posterior or concave surface. In plate V. fig. 4, the
enamel of the broken incisors is represented by short lines, showing the
direction of its crystalline fibres; the white space immediately within
the enamel shows the thickness of the ivory at the base of the tooth,
the included gray substance represents a section of the formative matrix
or pulp of the tooth, which was of the usual conical form: the inferior
broken end of the incisor (fig. 5,) appears to have been distant about
one-third from the apex of the pulp.
From the relative position of the bases or roots of these incisors, we
may infer that they diverged from each other as they advanced forwards,
in order to bring their broadest cutting surfaces into line. That they
were opposed to teeth of a corresponding structure in the upper jaw is
proved by the oblique chisel-like cutting surface of the more perfect
incisor: and it is not without interest to find that the presence of
_dentes scalprarii_ at the anterior part of the mouth has not been
necessarily limited to Mammalia of small size.
The position of the pulps of these incisors, in close proximity with the
anterior grinders, corresponds with the position of the pulps of the
incisors in the upper jaw of the _Toxodon_, and indicates, in
conjunction with the size of the pulps, that a considerable extent of
the inferior incisors was lodged in the substance of the anterior part
of the jaw. It is most likely that no vertically directed tooth would be
developed in the part of the jaw so occupied by the curved bases of the
incisors, and hence a diastema or toothless space would intervene
between the molars and incisors of this lower jaw, as in the upper jaw
of the Toxodon.
It is interesting, also, to observe, that as the deviations from the
Rodent type, which occur in the cranium of the Toxodon, are the same, in
some instances, as those which obtain in the Wombat; so we find a
corresponding deviation in the size and relative position of the
inferior incisors, which, as in the Wombat, terminate anterior to the
molar teeth, instead of extending backwards beyond the last grinder, as
in most of the true Rodents. The Capybara presents the nearest approach
to this structure, the pulps of the inferior incisors being situated
opposite the interspace of the first and second grinders.
The molar teeth, in this mutilated lower jaw, like those in the upper
jaw of Toxodon, had persistent pulps, as is proved by the conical cavity
at their base, as represented in fig. 3; they consequently required a
deep socket, and a corresponding depth of jaw to form the socket and
protect the pulps. In order to economise space, and to increase the
power of resistance in the tooth, and perhaps, also, to diminish the
effects of direct pressure on the highly vascular and sensible matrix,
we find the molars and their sockets are curved, but in a less degree
than those of the upper jaw of the Toxodon. They correspond, however,
with the superior molars of the Toxodon in the antero-posterior
diameter, in being small and simple at the anterior part of the jaw, and
by increasing in magnitude and complexity as they are situated more
posteriorly. They are, however, narrower from side to side; but
supposing them to belong to the Toxodon, it would agree in this respect
with most other large herbivorous mammalia;—the fixed surface for
attrition in the upper jaw being from obvious principles more extensive
than the opposed moveable surface in the lower jaw.
The _first_ grinder, in the lower jaw here described (Pl. V. fig. 2), is
of small size and simple structure, being surrounded with a coating of
enamel of uniform thickness, and without any fold penetrating the
substance of the tooth. It is more curved than any of the other molars,
and appears to have differed from the external incisor only in its
entire coating of enamel and direction of growth; it is interesting,
indeed, to find so gradual a transition, in structure, from molar to
incisive teeth, as this jaw presents; for the robust incisors may here
be regarded as representing molars simplified by the partial loss of
enamel, and with a change in their direction.
In the _second_ molar, we find an increase in the antero-posterior
diameter, and in the length of the tooth, and the enamel at the middle
of the outer side makes a fold which penetrates a little way into the
tooth; the line of enamel, on the inner side, is slightly concave and
unbroken.
The _third_ molar presents an increase of dimensions in the same
directions as the second; the enamel on the outer side of the tooth
presents a similar fold, but it is directed a little more backwards.
In the _fourth_ molar, besides a further increase of size, and a
corresponding but deeper fold of enamel in the external side of the
tooth, we have the grinding surface rendered more complicated by two
folds of enamel entering the substance of the tooth from the inner side:
these folds divide the antero-posterior extent of the tooth into three
nearly equal parts; they are both directed obliquely forwards, half-way
across the substance of the ivory.
The _fifth_ molar presents the same structure as the fourth, which it
exceeds only slightly in size.
In the _sixth_ molar we have a proportionally greater increase of size
in the antero-posterior diameter, which measures two inches; but the
lateral diameter is but slightly augmented; its structure resembles that
of the fifth.
As 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, but in a greater degree, the compressed form which
characterizes the grinders of the upper jaw of the Toxodon.
It will be seen, however, that there is a difference in the structure of
the grinders in this fragment of the lower jaw and those of the upper
jaw of the Toxodon. In the lower grinders there are two folds of enamel
proceeding from the inner side of the tooth into its substance, whilst
in the upper grinders there is only one fold continued from the inner
side; in the lower grinders there is also a fold of enamel reflected
into the substance of the tooth from the outer surface, while in the
upper grinders of Toxodon we find the enamel coating on the outer side
of the tooth merely bent inwards, so as to describe, in the transverse
section, a gently undulating line; fig. 7, Pl. V. is the grinding
surface of the sixth molar, right side, upper jaw.
But this difference of structure is by no means incompatible with the
co-existence of the two series of teeth in the same animal, since we
find the grinders of the upper and lower jaws presenting differences of
structure of equal degree in existing herbivorous species. If we examine
the jaws of the Horse, for example, we shall find not only an equal
amount of difference in the structure of 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 of the external enveloping layer of enamel,
leaving out of consideration the central crescentic islands of enamel
which constitute the additional complexity of the Horse’s grinder.
Viewing then the course of the external coat of enamel on the worn
surface of the tooth, we find it describing on the outer side of the
tooth in the upper jaw an undulating line,—a middle convexity being
situated between two concavities; on the inner side of the tooth one
fold of enamel penetrates to the middle of the tooth, and on each side
of this there is a smaller fold. But in the lower jaw the line of enamel
on the outer side of the tooth, instead of merely bending outwards
midway in its course, is reflected a little way inwards; while on the
opposite, or inner side of the tooth, the enamel sends two extensive
folds into the substance of the tooth, opposite to the interspace of
which the shorter fold projects from the outer side. Now, on the
supposition that the fragment of the lower jaw here described belongs to
the Toxodon, the kind and degree of difference in the complexity of the
grinding surface of the teeth in the upper and lower jaw, are remarkably
analogous to those which exist in the Horse. I have only further to
remark that in the Horse the inflected folds of enamel, instead of being
simple and straight with the two constitutive layers in apposition, as
in the Toxodon, are irregular in their course, with cœmentum intervening
between the constitutive layers, which also diverge from each other at
their angle of reflection, so as to augment the amount of dense material
which enters into the composition of the tooth.
Many analogous examples will readily occur to the experienced
comparative anatomist. The Horse has been adduced as one to which
reference can very readily be made; but I would also cite the Sumatran
Rhinoceros, the skull of which, in the Hunterian collection, has already
been alluded to. In this species the anterior grinders, in both jaws,
are small and simple, and increase in complexity as they recede
backwards. The third superior grinder (fig. 8, Pl. V.) presents a single
fold of enamel, reflected obliquely forwards from the inner side
half-way across the tooth; the outer line of enamel describes a simply
undulating line. The opposite grinder of the lower jaw (fig. 9, Pl. V.)
has only one-half the breadth of the upper one, but has its grinding
surface further complicated by having two inflected folds of 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 of the lower jaw of the Toxodon, but differs
essentially in being of limited growth, and consequently in having
fangs.[15]
In speculating upon the nature of the organized substances which the
teeth of the Toxodon were destined to grind down, we must not only take
the structure of the tooth into consideration, but also the power of
perpetual renovation, which will compensate for the defective quantity
of enamel in the grinders of the Toxodon, as compared with those of the
existing Ruminants and Pachyderms, whose grinders, when once completed,
receive no further addition of dental substance at their base. The
Toxodon, in this character of its dentition, participated in the same
advantages with the Capybara and the Megatherium.
Although we have been enabled to observe the structure of the grinding
teeth of the upper jaw of the Toxodon in two examples only; one, an
insulated perfect grinder corresponding to the sixth alveolus on the
right side, and the other, a portion of the last grinder of the left
side remaining in the socket of the head previously described, yet from
the relations subsisting between socket and tooth, a very satisfactory
opinion may be formed of the structure of those teeth which are wanting,
as well as of their size. It thus appears, that the grinders of the
upper jaw of the Toxodon, are small and simple at the anterior part of
the jaw, and that they increase (chiefly in antero-posterior extent) in
size, as well as in complexity, as they recede backwards in the jaw. In
this respect, as well as in size, the teeth, in the fragments of the
lower jaw just described, exactly correspond. There is, however, a
slight difference in the lateral diameter of the two sets of grinders,
those of the lower jaw being narrower, as is usually the case, but not
in the same degree as in the Horse or Ruminant. A greater difference
obtains in the degree of curvature of the two sets of molars, those of
the lower jaw, especially the posterior grinders, being much less bent
than the corresponding teeth of the upper jaw. It is necessary to
observe, also, that the convexity of the curve of the inferior grinders
is directed outwards, as in the superior grinders; while in the Guinea
Pig and Wombat, which have also curved grinders, the convexity is
outwards in the lower jaw, and inwards in the upper jaw.
Nevertheless, if we take into consideration the close similarity which
exists between the teeth of the upper jaw of the Toxodon, and those of
this lower jaw in more essential points, as in their persistent pulps,
their characteristic structure and form, the depth of their sockets, and
their relative sizes and complexity; and when we consider how the depth
of this lower jaw, and its narrowness in the transverse direction,
corresponds with the characteristic form of the upper jaw of the
Toxodon, and that to these resemblances is added an apparatus of
incisors adequate to oppose the great dentes scalprarii of the upper
jaw, the conclusion seems irresistible, that the lower jaw, here
described, must be referred, if not to the same, at least to a nearly
allied species of Toxodon, as that to which the large cranium belonged.
Further researches in South America, it is hoped, will lead, ere long,
to the completion of our knowledge of the osteology of this very
remarkable and interesting genus of extinct mammiferous animals.
DESCRIPTION OF PARTS OF THE SKELETON OF
MACRAUCHENIA PATACHONICA;
_A large extinct Mammiferous Animal, referrible to the Order
Pachydermata; but with affinities to the Ruminantia, and especially to
the Camelidæ_.
In the preceding pages the nature and affinities of a large extinct
Mammal were attempted to be determined from the cranium and teeth
exclusively: we come now to consider the remains of a quadruped
consisting of bones of the trunk and extremities, without a fragment of
a tooth or of the cranium to serve as a guide to its position in the
zoological scale.
It may appear, even to anatomists and naturalists familiar with the kind
of evidence afforded by a fossil fragment, that an opinion as to the
relation of the present species to a particular family of Ruminants,
formed without a knowledge of the important organs of manducation, must
be vague and doubtful, but the evidence about to be adduced, will be
regarded, it is hoped, as more conclusive than could have been _à
priori_ expected.
The portions of the skeleton of the animal—which, in relation to the
affinity above alluded to, as well as from the length of its neck, I
propose to call _Macrauchenia_[16]—were discovered by Mr. Darwin in an
irregular bed of sandy soil, overlying a horizontal accumulation of
gravel on the south side of Port St. Julian: and independently of the
circumstances under which they were found, their correspondence with
each other in size, colour, texture and general character prove them to
have belonged to one and the same individual.
These remains include two cervical vertebræ, seven lumbar vertebræ, all
more or less fractured; a portion of the sacrum and ossa innominata;
fragments of the right scapula; of the right radius and ulna, and right
fore-foot; the right femur nearly entire, the proximal and distal
extremities of the right tibia and fibula; and a metatarsal bone of the
right hind-foot.