anchylosed tibia and fibula have given of the correspondence of the
hind-foot with the fore-foot of the Macrauchenia, in regard to the
number of the toes, receives ample confirmation from the astragalus,
which, of all the bones in the foot, is the one that an anatomist would
have chosen, had his choice been so limited, and which most fortunately
has been secured by Mr. Darwin, in a very perfect state, in the present
instance. I have compared this astragalus with that of the Giraffe, and
other Ruminants, the Camel, the Anoplothere, the Horse, the Hog, the
Hippopotamus, Rhinoceros, Tapir, and Palæothere: it is with the
Pachyderms having three toes to the hind-foot, that the Macrauchenia
agrees in the main distinguishing characters of this bone; its anterior
articular surface, for example, is simple, and not divided into a double
trochlea by a vertical ridge: lastly, it is with the astragalus of the
Tapir and Palæothere that it presents the closest correspondence in the
general form and the minor details of structure, and with these
Pachyderms, therefore, I shall chiefly limit the comparison of the
Macrauchenia, in regard to the bone in question. If the upper or tibial
articular surface (fig. 5, Pl. XIV.) be compared with that in the
_Palæotherium magnum_ (Ossem. Foss. Pl. LIV. fig. 2,) it will be seen,
that the general direction of that surface is more parallel with the
axis of the bone in Macrauchenia. In the Palæotherium it is turned a
little towards the outer or fibular side, and in the Tapir the general
direction of the same surface is placed still more obliquely. The
anterior border of this articulating surface is broken by a semicircular
notch in the Palæothere; in the Tapir it describes a gentle concave
curve, and the Macrauchene resembles the Tapir in this respect. The
chief difference between the astragalus of the Tapir and the Palæothere,
when viewed from above, obtains in the relative length of the bone,
anterior to the tibial articulating surface: the Macrauchene presents,
in this respect, an intermediate structure, but differs from both in the
greater extent of the tibial side of this part of the astragalus.
If we next direct attention to the anterior or scaphoid articular
surface, (fig. 3, Pl. XIV.) and compare it with that of the
_Palæotherium magnum_, (fig. 4, Pl. LIV, Ossem. Foss.) it will be seen,
that it presents in the Macrauchenia an oval, and in the Palæotherium an
irregular quadrangular form: in the Macrauchenia, this surface is
uniform or undivided, and is gently convex, except at its lower part;
while in the Palæothere it is divided by an oblique ridge into a broad
internal facet for the scaphoid bone, and a narrow internal surface for
articulation with the os cuboides; the larger surface is also concave
transversely, and slightly convex vertically: in the Tapir, the anterior
surface of the astragalus deviates still further from that of the
Macrauchenia, both in general form, and in the proportion of the
cuboidal facet. In the didactyle Anoplotherium, Camel, and true
Ruminants, where the cuboides presents a large relative size, a still
greater proportion of the anterior surface of the astragalus is devoted
to the articulation with this bone, and is separated from the scaphoid
surface by a well-developed vertical ridge. The Macrauchenia presents,
therefore, the extreme variation from this type;—and should the entire
tarsus hereafter be discovered, it will doubtless be found, that the os
cuboides is articulated posteriorly to the os calcis exclusively.
The external surface of the astragalus of the Macrauchene, (fig. 1. Pl.
XIV,) is longer in proportion to its vertical extent than in the Tapir
or Palæothere: the articular surface for the fibular malleolus is less
curved. Between this surface and the anterior facet the bone is
excavated by a deep notch, both in the Tapir and Palæothere; but in the
Macrauchenia by a gentle concavity. Beneath the malleolar articular
smooth surface in the Palæothere there is a deep pit; in the Tapir a
shallow one; but in the Macrauchenia we observe only a smooth and
slightly convex triangular surface. If we compare the inner surface of
the astragalus in these three animals, we shall find the existing Tapir
again forming a transition between the two extinct genera. In the
Palæothere, a round protuberance projects from the anterior part of this
surface: in the Tapir, we observe a gentle rising of the bone in the
same part, while in the Macrauchene (fig. 2) the surface of the bone is
level at this part. The margin of the tibial malleolar articular
surface, which is very slightly raised in the Macrauchene, is more
developed in the Tapir, and still more so in the Palæothere, where it
forms a ridge, overhanging the rough outer side of the bone. Near the
lower part of this surface we observe a small but deep depression in the
Palæothere; there is a shallower one in the corresponding part in the
Tapir; and the depression is still wider and shallower in the
Macrauchenia. In the Palæothere the astragalus articulates by three
surfaces with the os calcis, posteriorly by a large concave surface,
externally by a longitudinal sub-elliptic surface, and anteriorly by a
thin transverse facet: in the Macrauchene (fig. 4) two only of these
surfaces are present, viz. the concave and the longitudinal one, the
anterior transverse surface being wanting: in the Tapir, the transverse
surface is present, but is confluent with the longitudinal one. The
posterior surface is relatively larger and deeper in the Macrauchene
than in the Palæothere, and approaches nearer to the triangular than the
oval form: the longitudinal surface is placed more obliquely, and is
truncated anteriorly. In the Tapir this surface is confluent with the
scaphoid articular surface, but it is separated therefrom by a narrow
strip of bone in both the Palæothere and Macrauchene. It is satisfactory
to find in the bone, which marks most strongly the affinity of
_Macrauchenia_ to _Palæotherium_, so many easily recognizable
differences, because the structure of the cervical vertebræ in the
latter genus is too imperfectly known, to allow us to predicate
confidently a distinction between it and _Macrauchenia_ in that
particular; the difference, however, which they present in the condition
of the bones of the fore-arm and leg, forbids their being considered as
generically related.
There remains to be noticed only a single fractured metatarsal bone
(fig. 1. Pl. XV.) This, from its bent and unsymmetrical figure, is
evidently not a middle one, and having the side of the proximal end,
which was articulated to the adjoining metatarsal in a nearly perfect
state, it enables us to refer it with certainty to the hind-foot, since
it does not agree with any of the corresponding surfaces at the proximal
extremities of the metacarpal bones. It remains then to be determined,
whether it is an external metatarsal of the right-foot, or an internal
one of the left-foot, the general curvature of these being in the same
direction. With neither of these bones in the Tapir does our metatarsal
agree, since it has but one articular facet on the lateral surface of
its proximal end, while the outer metatarsal of the right-foot of the
Tapir, with which, in other respects, it most closely corresponds, has
two articular surfaces. In the cast of a hind-foot of a Palæothere, I
find that the outer metatarsal bone closely agrees with this metatarsal
bone of the Macrauchene, in the structure just alluded to: the
articulation with the middle metatarsal being by a single sub-oval
facet, which stands out a little way from the surface of the bone: the
articular surface in the Macrauchene presents a similar form and
condition, and is similarly situated to that in the Palæothere, being at
the posterior part of the lateral surface, and a little below the
superior or tarsal articular surface. The bone expands towards its
distal end, which corresponds in structure with those of the two lateral
metatarsals in the fore-foot, in being completely divided into two
trochlear surfaces by a well-developed median ridge, and in having the
posterior half of this ridge suddenly produced, so as to project about
two lines further from the trochlear surface than the anterior part of
the same ridge. In both the Tapir and Palæothere this anterior part of
the ridge is wholly suppressed, and the posterior is much more feebly
developed than in the Macrauchenia. The metatarsal bone here described
is of exactly the same length with the internal metacarpal bone, and
proves, in conjunction with the proportions of the astralagus, that the
fore and hind-feet of the Macrauchenia were of equal size.
Thus then we obtain evidence, from a few mutilated bones of the trunk
and extremities of a single representative of its race, that there once
existed in South America a Pachydermatous quadruped, not proboscidian,
which equalled in stature the Rhinoceroses and Hippopotamuses of the old
world. But this, though an interesting and hitherto unsuspected fact, is
far from being the sum of the information which is yielded by these
fossils. We have seen that the single ungueal phalanx bespeaks a
quadruped of the great series of _Ungulata_, and this indication is
corroborated by the condition of the radius and ulna, which are fixed
immoveably in the prone position. Now in the Ungulated series there are
but two known genera,—the Rhinoceros and Palæotherium,—which, like the
quadruped in question, have only three toes on the fore-foot. Again, in
referring the Macrauchenia to the Tridactyle family of Pachyderms, we
find, towards the close of our analysis, and by a detailed comparison of
individual bones, that the Macrauchenia has the closest affinity to the
Palæotherium.
But the Palæotherium, like the Rhinoceros and Tapir, has the ulna
distinct from the radius, and the fibula from the tibia; so that even if
the Parisian Pachyderm had actually presented the same peculiarities of
the cervical vertebræ as the Patagonian one, it would have been
hazardous, to say the least, while ignorant of the dentition of the
latter, to refer it to the genus _Palæotherium_.
Most interesting, indeed will be the knowledge, whenever the means of
obtaining it may arrive, of the structure of the skull and teeth in the
Macrauchenia. Meanwhile, we cannot but recognise, in the anchylosed and
confluent state of the bones of the fore-arm and leg, a marked tendency
in it towards the Ruminant Order, and the singular modifications of the
cervical vertebræ have enabled us to point out the precise family of
that order, with which the Macrauchenia is more immediately allied.
In first demonstrating this relationship, it was shown in how many
particulars the _Camelidæ_, without losing the essential characters of
Ruminantia, manifested a tendency to the Pachydermatous type; and the
evidence which the lost genera, _Macrauchenia_ and _Anoplotherium_, bear
to a reciprocal transition from the Pachyderms to the Ruminants, through
the _Camelidæ_, cannot but be viewed with extreme interest by the
Zoologist engaged in the study of the natural affinities of the Animal
Kingdom.
The Macrauchenia is not less valuable to the Geologist, in reference to
the geographical distribution of animal forms. It is well known how
unlooked-for and unlikely was the announcement of the existence of an
extinct quadruped entombed in the Paris Basin, whose closest affinities
were to a genus, (_Tapirus_,) at that time, regarded as exclusively
South American. Still greater surprise was excited when a species of the
genus _Didelphys_ was discovered to have co-existed in Europe with the
_Palæotherium_.
Now, on the other hand, we find in South America, besides the Tapir,
which is closely allied to the Palæothere,—and the Llama, to which the
Anoplothere offers many traces of affinity,—the remains of an extinct
Pachyderm, nearly akin to the European genus _Palæotherium_: and,
lastly, this Macrauchenia is itself in a remarkable degree a
transitional form, and manifests characters which connect it both with
the Tapir and the Llama.
ADMEASUREMENTS OF THE BONES OF THE MACRAUCHENIA.
Inches. Lines.
Length of third (?) cervical vertebra 7 9
Vertical diameter of ditto 4 0
Vertical diameter of body of ditto 2 3
Transverse diameter of ditto 3 3
Vertical diameter of spinal canal 1
Length of fourth lumbar vertebra 5 5
Vertical diameter of body of ditto 2 9
Transverse diameter of ditto 2 10
Vertical diameter of spinal canal 1 1
Transverse ditto ditto[26] 1 6
Transverse diameter of last lumbar vertebra 9
Transverse diameter of body of ditto 2 2
Vertical diameter of ditto 1 3
Entire length of lumbar region of vertebral column 20
Vertical diameter of glenoid cavity of scapula 3
Transverse ditto ditto ditto 2 10
Elevation of spine of scapula 3 5
Vertical diameter of proximal articular surface of
fore-arm 3 6
Transverse ditto ditto ditto 3 5
Height of olecranon 5 3
Greatest diameter of its base 2
Circumference of proximal end of anchylosed radius and
ulna 11 10
Entire length of inner toe of fore-foot, inclusive of
metacarpal bone 13
Breadth of proximal end of metacarpus 3 8
Breadth of distal end of ditto 5 4
Length of inner metacarpal bone 7 6
Length of middle ditto 8
Length of outer ditto 7
Length of inner proximal phalanx 3 6
Length of middle ditto 2 10
Length of outer ditto 3 4
Length of inner middle phalanx 2
Length of middle ditto 2 3
Length of inner distal phalanx[27] 1
Length of the femur 24
Diameter of base of articular surface of the head of
ditto 3 6
Greatest diameter of proximal end 7
Greatest diameter of distal end 6 3
Circumference of middle of shaft 8
Length of tibia 18
Greatest diameter of proximal end 5 7
Greatest diameter of distal end, including fibula 4 4
Circumference of middle of shaft 9
Length of metatarsal bone[28] 7 4
ERRATA.—The reader is requested to substitute the word ‘_right_’ for
‘left’ in the last line of p. 35, before the words ‘radius,’
‘fore-foot,’ and ‘femur,’ and in the first line of p. 36,
before the words ‘tibia,’ and ‘hind-foot.’
DESCRIPTION OF A FRAGMENT OF A CRANIUM OF AN EXTINCT MAMMAL, INDICATIVE
OF A NEW GENUS OF EDENTATA, AND FOR WHICH IS PROPOSED THE NAME OF
GLOSSOTHERIUM.
“La première chose à faire dans l’étude d’un animal fossile, est de
reconnaitre la forme de ses dents molaires; on détermine par-là s’il est
carnivore ou herbivore;” says Cuvier, at the commencement of that series
of splendid chapters in which the restoration of the extinct Pachyderms
of the Paris Basin is recorded. In the present case, however, as in that
of the Mammiferous animal whose fossil remains we were last considering,
the important organs, to which Cuvier directs our first attention, are
wanting. Nor are there here, as in the Macrauchenia, any remains of the
locomotive extremities to compensate for the deficiency of teeth, and
guide us into the right track of investigation and comparison. The
animal, the nature and affinities of which are the subject of the
following pages, is, in fact, represented in Mr. Darwin’s collection, by
nothing more than a fragment of the cranium.
This fragment, which was found in the bed of the same river, (see p.
16,) in Banda Oriental, with the cranium of the Toxodon, includes the
parietes of the left side of the cerebral cavity, the corresponding
nervous and vascular foramina, the left occipital condyle, a portion of
the left zygomatic process, and, fortunately also, the left articular
surface for the lower jaw. The importance of this surface in the
determination of the affinities of a fossil animal has been duly
appreciated, since the relations of the motions of the lower jaw to the
kind of life of each animal were pointed out by Cuvier; but yet we
should be deceived were we to establish, in conformity with the
generalization enunciated by Cuvier,[29] our conclusion, from this
surface, of the nature of the food of the extinct species under
consideration; for the glenoid cavity is so shaped as to allow the lower
jaw free motion in a horizontal plane, from right to left, and forwards
or backwards, like the movements of a mill-stone; and, nevertheless, I
venture to affirm it to be most probable, that the food of
_Glossotherium_ was derived from the animal and not from the vegetable
kingdom; and to predict, that when the bones of the extremities shall be
discovered, they will prove the Glossothere to be not an ungulate but an
unguiculate quadruped, with a fore-foot endowed with the movements of
pronation and supination, and armed with claws, adapted to make a breach
in the strong walls of the habitations of those insect-societies, upon
which there is good evidence in other parts of the present cranial
fragment, that the animal, though as large as an ox, was adapted to
prey.
We perceive, in the first place, looking upon the base of this portion
of skull, a remarkable cavity, situated immediately behind the tympanic
bone, of nearly a regular hemispherical form, an inch in diameter (fig.
2, _b_, Pl. XVI). The superficies of this cavity appears not to have
been covered with articular cartilage, for it is irregularly pitted with
many deep impressions; and I conclude, therefore, that it served to
afford a ligamentous attachment to the styloid element of a large _os
hyoides_. With this indication of the size of the skeleton of the
tongue, is combined a more certain proof of the extent of its soft, and
especially its muscular parts, in the magnitude of the foramen, for the
passage of the lingual or motor nerve (_c._ fig. 2 and 3). This foramen,
(the anterior condyloid,) in the present specimen, is the largest of
those which perforate the walls of the cranium, with the exception of
the foramen magnum; it is fully twice the size of that which gives
passage to the second division of the fifth nerve; its area is oval, and
eight lines in the long diameter, so that it readily admits the passage
of the little finger.
It is only in the Ant-eaters and Pangolins that we find an approximation
to these proportions of the foramen for the passage of the muscular
nerve of the tongue; and the existing Myrmecophagous species even fall
short of the larger fossil in this respect. Some idea of the size of the
lingual nerve, and of the organ it was destined to put in motion, may be
formed, when it is stated that the foramen giving passage to the
corresponding nerve in the Giraffe,—the largest of the Ruminants, and
having the longest and most muscular tongue in that order,—is scarcely
more than one-fourth the size.
With these indications of the extraordinary development of the tongue,
we are naturally led, in order to carry out a closer and more detailed
comparison of the fossil in question, to that group of mammalia in which
the tongue plays the chief part in the acquisition of the food. The
size, form, and position of the occipital condyle,—the magnitude of the
occipital foramen, (which must here have somewhat exceeded three inches
in the transverse diameter,)—the slope of the occipital surface of the
cranium from below, upwards and forwards, at an angle of 60° with the
base of the cranial cavity-each and all attest the close affinities of
the present animal to the Edentata. More decisive evidence of the same
relationship will be adduced from the organization of other parts of the
cranium. The glenoid articular surface (_a_, fig. 2, Pl. XVI.) is an
almost flattened plane, wider in the transverse than in the longitudinal
direction; and, as in the genera _Myrmecophaga_ and _Manis_, it is not
defended behind by any descending process. In its general form it
resembles the glenoid cavity of _Orycteropus_ more than that of the
preceding Edentates; but, in _Orycteropus_, the articulation is defended
posteriorly by a descending process of the zygoma, and it is also
situated relatively closer to the os tympanicum.
Had the _Glossotherium_ teeth? The extent of the temporal muscle, which
is indicated by the rugged surface of the temporal fossa, and by the
well-marked boundary, formed by a slightly elevated bony ridge, which
extends to near the line of the sagittal suture, together with the size
of the zygomatic portion of the temporal bone, and the remains of the
oblique suture by which it was articulated to the malar bone, enables me
to answer this question confidently in the affirmative. They will
probably be found to be molar teeth of a simple structure, as in the
Orycteropus.
The evidence just alluded to of the existence of an os malæ is
interesting, because this bone is wanting in the Pangolins; and its
rudimental representative in the true Ant-eaters does not reach the
zygomatic process of the temporal bone, which consequently has no
articular or sutural surface at its anterior extremity. In the presence,
therefore, of the surface for the junction of the os malæ, and the
consequent evidence of the completion of the zygomatic arch, we learn
that the Glossothere was more nearly allied to the Armadillos and
Orycterope. That its affinity to the latter genus was closer than to the
Armadillos we have most interesting evidence in the form and loose
condition of the tympanic bone: it is represented of the natural size at
fig. 4, Pl. XVI. Through the care and attention devoted to his specimens
by their gifted discoverer, this bone was preserved _in situ_, as
represented at _d_, fig. 1; but it had no osseous connection with the
petrous or other elements of the temporal bone, and could be displaced
and replaced with the same ease as in the _Orycterope_. This bony frame
of the membrana tympani, in the Glossothere, describes rather more than
a semicircle, having the horns directed upwards; it has a groove, one
line in breadth, along its concave margin, for the attachment of the
ear-drum, and sends down a rugged process, half an inch long, from its
lower margin. In the _Dasypodes_ and _Myrmecophagæ_, the tympanic bone
soon becomes anchylosed with the other parts of the temporal; it is only
in _Orycteropus_, among the existing insectivorous _Bruta_ or
_Edentata_, that it manifests throughout life the fœtal condition of a
distinct bony hoop, deficient at the upper part. The os tympanicum of
_Orycteropus_, however, differs from that of _Glossotherium_, in forming
part of the circumference of an ellipse, whose long axis is vertical;
and in sending outwards, from its anterior part, a convex eminence,
which terminates in a point directed downwards and forwards.
Such appear to be the most characteristic features of the cranial
fragment under consideration, in which we have found, that the articular
surface for the os hyoides throws more light upon the nature of the
animal of which it is a part, than even the glenoid cavity itself. There
now remains to be described as much of the individual characters of the
constituent bones as the specimen exhibits.
The occipital bone, besides forming the posterior and part of the
inferior parietes of the cranium, extends for about half an inch upon
the sides, where the ex-occipital element is articulated by a vertical
suture with the mastoid element of the temporal: this suture is situated
in a deep and well-marked muscular depression (_e_, fig. 1), measuring
three inches in the vertical, and upwards of one inch in the transverse
direction. The other sutures, uniting the occipital to the adjoining
bones, are obliterated. The breadth of the occipital region must have
exceeded the height of the same by about one-third. The condyle extends
nearly to the external boundary of the occipital aspect of the cranium;
there is situated, external to it, only a small ovate, rounded and
smooth protuberance. The slightly concave surface of the occipital plane
of the cranium is bounded above by a thick obtuse ridge, the muscular
impressions are well sculptured upon it. It is traversed transversely at
its upper third by a slightly elevated bony crest; and the surface below
this ridge is again divided by a narrower intermuscular crest, which
runs nearly vertically, at about an inch and a half from the external
boundary of the occipital plane. As a similar crest must have existed on
the opposite side, the general character of the occipital surface in the
Glossothere would resemble that of the Toxodon. A similar correspondence
may be noticed in the terminal position of the condyle, and the slope of
the occipital plane.