fragments of a left scapula; the proximal extremities of the anchylosed
bones of the right antibrachium; the metacarpal and most of the
phalangeal bones of the right fore-foot. The first-mentioned fragments,
include the head and neck of the scapula, a small part of its body with
the beginning of the spine, the coracoid process, and the nearly entire
glenoid cavity. This articular surface (fig. 2, Pl. IX.) resembles in
its general form, and degree of concavity, that of the Camel and
Rhinoceros, and is deeper than in the Hippopotamus. The coracoid process
is represented by a slightly produced rough, thick, and obtuse
tuberosity, situated closer to the glenoid cavity than in the _Camelidæ_
or _Rhinoceros_, and having almost the same relative position and size,
as in the _Palæotherium crassum_. The superior border or costa of the
scapula presents much variety in the Ungulate quadrupeds with which we
have to compare the Macrauchenia. In the Ruminants its contour forms
behind the coracoid a concave sweep, which advances close to the spine
of the scapula. In the Camel and Horse the marginal concavity is
shallower, and the distance of the superior costa from the spine of the
scapula is greater; the extent of the supra-spinal fossa increases in
the true Pachyderms, and the Macrauchene agrees with them in this
structure. In the Tapir, however, the contour of the superior costa is
broken by a deep round notch immediately behind the coracoid: in the
Hippopotamus this process arches in a slight degree backward over a
corresponding but wider and shallower notch. In the _Palæotherium
crassum_ the concavity of the superior costa, behind the coracoid, is as
slight as in the Rhinoceros; but in the Macrauchenia the superior costa
of the scapula begins to rise or stretch away from the parallel of the
spine, immediately behind the coracoid process. The modifications of the
spine of the scapula which characterize respectively the Ruminants and
Pachyderms have been clearly and concisely set forth by Cuvier, who at
the same time points out the exceptional condition which the _Camelidæ_
present in the production of the acromial angle. It was with peculiar
interest and care, therefore, that I reunited all the fragments of the
scapula of the Macrauchene, in the hope of gaining from this part of the
skeleton as decisive evidence of an affinity to the Camel as the
cervical vertebræ had afforded. It unfortunately happens, however, that
the part of the scapula most important in this comparison is broken off;
yet from this very circumstance, combined with a slight inclination
forwards of the anterior margin of the spine immediately beneath the
fractured acromion, and from the thickness of the fractured surface, we
may infer that the acromial angle of the spine was more produced than in
the ordinary Ruminants, although evidently in a less degree than in the
Camel tribe. The Macrauchenia, however, surpasses these aberrant
Ruminants, and equals the Pachyderms in the elevation and extent of its
scapular spine: but this process commences about half an inch behind the
glenoid cavity, and rises at once to the height of three inches above
the plane of the scapula; in which structure we may trace the same
tendency to the Ruminant type, as is manifested in the scapula of the
Hippopotamus and Anoplotherium; for in most other Pachyderms the spine
increases gradually from its extremities to the middle part. The
anterior margin of the spine beneath the short acromion is perforated by
an elliptical fissure measuring ten lines, by three lines. The extent of
the spine which is preserved, measures eight inches and a half; it is a
thin and nearly straight plate of bone, expanding into a thick and
rugged upper margin, which slightly over-arches the inferior fossa.
(fig. 1, Pl. IX.) In its general form and proportions the spine of the
scapula in Macrauchenia presents the nearest resemblance to that of the
Hippopotamus; but its origin is closer to the articular surface of the
scapula than in this, or any other Pachydermal or Ruminant genus.
The portion of the antibrachium of the Macrauchenia which is preserved,
presents a condition of the radius and ulna intermediate to those which
respectively characterize the same bones in the Pachyderms and Camels.
In the former the radius and ulna are separate bones, united in the
prone position by ligament, yet so that the movement of supination
cannot be performed; in the ordinary Ruminants they are partially joined
by bony confluence, which rarely extends to the proximal extremities; in
the Camel and Llama the anchylosis of the radius and ulna is so
complete, that no trace of their original separation can be perceived,
and the olecranon appears but as a mere process of the radius.
In the Macrauchenia the anchylosis of the radius and ulna is also
complete, but the boundary line of the two originally distinct bones is
very manifest, and the proportion which each contributes to the great
articulating surface for the distal end of the humerus is readily
distinguishable. About a sixth part of this surface is due to the head
of the radius, which enters into the composition of the anterior and
outer part of the articulation, and its extent is defined by a depressed
line describing a pretty regular curve, with the concavity directed
forwards and a little outwards. (_a_, fig. 1, Pl. X.) Just below the
articular surface a strong triangular rugged protuberance projects from
the front of the head of the radius, for the attachment of the tendon of
the biceps. The line of separation of the radius and ulna is indicated
on the inner side of the head of the radius by a deep and narrow fissure
extending downwards from below the anterior part of the articulating
surface; and on the outer side by a broad groove leading upwards to a
deep pit near the proximal end of the antibrachium. We may see by the
direction of the head of the radius which is thus defined, that it
crosses obliquely in front of the ulna, as in the Elephant,
Hippopotamus, and other Pachyderms, and that the bones are anchylosed in
the prone condition: below this fissure and groove, which mark the
interosseous line, the radius and ulna become blended together into one
compact bone, which is flattened from before backwards, with a well
marked ridge on the outer side; and excavated by a single medullary
cavity, the compact walls of which present a general thickness of
one-third of an inch.
The proximal articular surface or sigmoid cavity of the antibrachium,
constituted as above described, resembles that of the Palæothere, Tapir,
and the generality of the Pachyderms in having two depressions, instead
of three, as in the Anoplothere, and Ruminants. The Hippopotamus has a
slight tendency to the latter structure, which is also less marked in
the Camel than in the ordinary Ruminants. In its general form the
sigmoid cavity of the Macrauchene resembles that of the Hippopotamus
more than that of the Camel. In the Camel this articular surface is
traversed transversely by a broad, shallow, and slightly roughened
tract, which divides the smooth surface of the joint into two parts, one
forming the anterior horizontal surface due to the conjoined radius and
ulna, the other forming the vertical concave surface on the anterior
part of the base of the olecranon. In the Hippopotamus there is, as it
were, an attempt at a similar division of the articulating surface at
the proximal end of the antibrachial bones; a deeper and rougher
depression encroaches upon the articulation from its outer side, but
stops when it has reached half-way across. In the Macrauchenia the
roughened surface, (_b_. fig. 1, Pl. X.) commencing also at the outside,
extends only one-third of the way across the articular surface: it is,
however, as shallow as in the Camel. The articular surface on the
anterior part of the base of the olecranon is broader in the
Hippopotamus than in the Camel; but in the Macrauchene it is twice as
broad as in the Hippopotamus. The size of the olecranon in the
Macrauchene exceeds that of the Hippopotamus, and _à fortiori_ that of
the Camel: indeed in its general magnitude the Macrauchenia must have
fully equalled the largest Hippopotamus; but it no doubt had a more
shapely, and less broad and bulky trunk. The olecranon of the
Macrauchenia differs in shape, both from that of the Camel and
Hippopotamus; it terminates above in a three-sided cone with an obtuse
apex; and presents a well-marked protuberance at the outer side of the
base, which is not present in either the Camel or Hippopotamus. There is
also a strong rugged ridge on the back part of the olecranon which makes
an angle before sinking into the level of the ulna below.
The confirmation of the close affinity of the Macrauchenia to the
Pachydermatous Order, which the structure of the cervical vertebræ alone
might have rendered very doubtful, is afforded by the bones of the right
fore-foot (Pl. XI.); these are fortunately in so perfect a condition, as
to make it certain that this interesting quadruped had three toes on the
fore-feet, and not more; and that the fully developed metacarpal bones
are distinct, and correspond in number with the toes, and are not
anchylosed into a single cannon bone, as in the Ruminants. The bones
preserved are the metacarpals, proximal phalanges, and middle phalanges
of each of the three toes, and the distal phalanx of the innermost toe.
The proximal end of the innermost metacarpal bone presents three
articular surfaces; the middle facet is the largest, and the two lateral
ones slope away from it at an angle of 45°. The middle facet is broad
and slightly convex in front, narrow and concave behind; the distal
articular surface of the trapezoides must have corresponded with this
surface; the outer facet is narrow, flat, extends from the fore to the
back part of the head of the bone, and must have been adapted to a
corresponding surface on the os magnum; the inner facet is the smallest,
presents a triangular form, and is situated towards the back part of the
head of the metacarpal bone; it indicates the existence of a rudimental
metacarpal bone, or vestige of a pollex. Below the outermost of the
lateral surfaces there is a crescentic articular surface with its
concavity directed outwards and downwards (fig. 2, Pl. XV.), against
which a corresponding convex articular surface of the middle metacarpal
abuts, (fig. 3, Pl. XV.) External to this surface the proximal end of
the middle metacarpal bone presents two articular surfaces for the
carpus; the larger one, which was adapted to the os magnum, is
horizontal, broad and convex before, narrow and concave behind; the
outermost facet is a small triangular surface inclined downwards to the
level of the articulating surface of the outermost metacarpal. It also
presents a posterior vertical articular surface for a sesamoid bone. The
proximal extremity of the outer metacarpal bone is joined to the middle
metacarpal, not by one semilunar surface, but by two separate
articulations of small size (fig. 4 and 5, Pl. XV.); it presents a
single large slightly convex articular surface for the os magnum, of an
irregular semicircular form, with the convexity of the curve turned
outwards.
The metacarpus increases in breadth as it approaches the phalanges; the
two lateral metacarpals bending slightly away from the middle one, and
expanding towards their distal extremities: the middle bone presents a
symmetrical figure except at its proximal extremity (fig. 2, Pl. XI.)
The distal articulating facet of each of the metacarpal bones extends so
far upon both the anterior and posterior surfaces as to describe more
than a semicircle (fig. 3, Pl. XI.); in the two lateral metacarpals it
is traversed throughout by a longitudinal convex ridge dividing it into
two equal lateral parts; the ridge is most produced on the posterior
half of the joint (fig. 4, Pl. XI.): in the middle metacarpal this ridge
subsides before it reaches the anterior part of the articular surface.
The proximal extremity of the middle proximal phalanx presents a
posterior notch corresponding to the above partially developed ridge:
the proximal extremities of the lateral phalanges are traversed by a
middle longitudinal depression, and two lateral shallow concavities
(fig. 6, Pl. XI.); but these are of such an extent as to be in contact
with only a part of the convexity above, which therefore was doubtless
adapted to a sesamoid bone on each side of the longitudinal ridge. The
structure of the above described joints proves that the motion of the
toe upon the metacarpus was much freer and more extensive than in the
Rhinoceros, which is the only existing Ungulate mammal which presents
the tridactyle structure in the fore-foot. In this species the
metacarpo-phalangeal articulations exhibit only a slight trace of the
longitudinal ridges and grooves which are confined to the posterior part
of the joint; these are more developed in the _Camelidæ_; but the Hog
and Horse in this respect approach nearer to the Macrauchene, though the
structure of the metacarpo-phalangeal joints in the Hog falls far short
of the compactness and strength combined with freedom of play in flexion
and extension which distinguish those of the Macrauchene. The
_Palæotherium medium_ most resembles the Macrauchene in the structure of
the trochlear metacarpo-phalangeal joints; but both in this species,[23]
and the _Pal. crassum_[24] the articular surface at the distal end of
the metacarpal is relatively narrower than in the Macrauchenia; moreover
all the species of the extinct Palæothere differ from the Macrauchene in
the greater size and strength of the middle as compared with the lateral
metacarpals.
The articulation at the distal extremity of the proximal phalanges (fig.
5, Pl. XI.) is simple, and not divided into two pulleys by a
longitudinal ridge; it is slightly concave from side to side; but in its
extent upon the anterior and posterior surfaces of the bone indicates a
freedom of flexion and extension of the toes, which harmonizes with the
structure of the joint above.
The proximal articulating surfaces of the second phalanges (fig. 7, Pl.
XI.) corresponds of course to those to which they are adapted; they are,
however, characterized by sending upwards an obtuse process from the
middle of their anterior margin. The distal articulating surfaces (fig.
8, Pl. XI.) resemble those of the proximal phalanges, but extend further
upon the back part of the phalanx than the front, indicating the more
horizontal position of the second phalanges.
The last phalanx, does not resemble the neatly defined ungulate
phalanges of the Ruminantia, and Solipedia, but has the irregular form
characteristic of those of the Pachydermata. It is wedge-shaped, broader
than it is long, with a rugged surface, except where it plays upon the
distal end of the second phalanx, where it is slightly concave in one
direction, and convex in the other, (figs. 7 and 9, Pl. XI.) A portion
of this phalanx extends backwards behind the articular surface, as in
the corresponding bone of the Palæothere and Rhinoceros.
The femur of the Macrauchenia (fig. 1, Pl. XII.) is full two feet in
length, and consequently longer than in any known Camel or Rhinoceros;
as compared with its transverse diameter it is much longer than the
femur of the latter animal: in the proportion of its breadth to its
length, and the expansion of its extremities as compared with the
diameter of the shaft, it more resembles that of the Camel. The femur of
the Giraffe deviates from that of the Macrauchenia in the excessive
expansion of its distal extremity. But the most striking evidence
deducible from this bone, of the affinity of the Macrauchenia to the
true Pachydermatous type is afforded by the evident traces of a third
trochanter, the outline of which is conjecturally restored in the
figure. Of the Pachyderms which have this characteristic structure, the
extinct Palæothere offers the nearest resemblance to the Macrauchene in
the general form and structure of the femur.
The head of the femur in the Macrauchene (fig. 2, Pl. XII.) presents the
form of a pretty regular hemisphere; it is less flattened above, and is
directed more obliquely inwards than in the Palæothere: the neck
supporting it does not project so far from the shaft as in the
Palæothere or Tapir, but farther than in the Camel. The great trochanter
rises above the level of the head; in which structure and in the
depression between the head and trochanter, the femur of the Macrauchene
offers a character intermediate between the Tapir or Palæothere, and the
Camel. The lesser trochanter is a slight projection from a ridge of bone
which is continued from the under part of the head of the femur to the
inner surface of the shaft. In the Palæothere the lesser trochanter is
situated more towards the posterior surface of the femur; so that, in
this particular, the Macrauchene approaches nearer to the Camel. Cuvier
makes no mention of the condition of the depression for the _ligamentum
teres_ in the Palæothere. Among existing ordinary Pachyderms the
Hippopotamus presents no trace of the insertion of a _ligamentum teres_
in the head of the femur; in the Camel the place of its insertion is
indicated by a well-marked circumscribed pit; in the Tapir a similar
circular depression is situated close to the inferior margin of the
articular convexity. The ligament was undoubtedly present in
Macrauchenia, but the place of its insertion is a broad and deep notch
leading from the under and back part of the head of the bone a little
way into its articular surface: this I regard as another of those
interesting transitional structures with which the remains of the
Macrauchenia, few and imperfect though they unfortunately are, so freely
abound.
The femur of Macrauchenia, in the flatness of the back part of its neck,
and the elongated form of the post-trochanterian depression, resembles
that of the Camel rather than that of the Palæothere; and the same
resemblance is shown in the cylindrical figure, straightness, and length
of the shaft. The depth of the trochanterian depression, and the
incurvation of the strong ridge continued downwards from the great
trochanter are individual peculiarities in the Macrauchenia.
A great part of the third trochanter is broken off; but from the remains
of its base we see that it had the same relative size as in the
Palæothere; but it is situated at the middle of the shaft of the femur,
and consequently lower down than in the Palæotheres and Tapirs. In the
general form and relative size of the condyles at the distal extremity
of the femur (fig. 3, Pl. IX. and XII.) the Macrauchene is intermediate
to the Camel and Palæothere, but resembles more the latter. In the
articular surface for the patella, it deviates somewhat from the
Palæothere, having this part longer in proportion to its breadth, more
regularly and deeply concave from side to side, and with its lateral
boundaries more sharply defined. In all these points the Macrauchene
approaches the Camel: the same affinity is shown in the protuberance
above the inner condyle; but in the extent of the posterior projection
of this condyle (fig. 3, Pl. IX.) it exceeds the Camel and Palæothere,
and displays an intermediate structure between these species and the
Hippopotamus.
There is a rough crescentic depression above the outer condyle where the
linea aspera begins to diverge; the corresponding depression is deeper
in the Hippopotamus, while in the Camel it is represented by a roughened
surface only, which is not depressed. In the fossa between the rotular
articulation and the external condyle the Macrauchene resembles the
Camel: the interspace of the condyles is relatively wider than in the
Camel, and the process above the inner condyle is more angular; in both
these respects the Macrauchene inclines towards the Palæothere.
In the structure of the bones of the leg of the Macrauchenia we find the
same transitional character which is afforded by the definable limits of
the anchylosed bones of the fore-arm. In the Pachyderma the fibula is an
entire and distinct bone. In the Ruminantia, with the exception of the
small Musk-deer, and, in an inferior degree, the Elk, the fibula appears
only as a short continuous process sent down from the under part of the
external condyle of the tibia. In the Camel tribe the only trace of the
fibula in the bones of the leg, is this process in a still more
rudimental state. In the Macrauchenia the fibula is entire, but is
confluent with the tibia through nearly its whole extent: the proximal
part of the fibula is well defined; its head is anchylosed to the outer
condyle of the tibia, but the shaft is continued free for the extent of
nearly two inches, and then again becomes confluent with the tibia,
forming apparently the outer ridge of that bone. About five inches from
the distal end of the tibia this outer ridge becomes flattened by being,
as it were, pressed against the tibia, and the anterior and posterior
edges are raised above the level of the tibia; beyond this part the
limits of the fibula begin again to be defined by deep vascular grooves.
The outer side of the distal end of the fibula is excavated by a broad
tendinous groove. The fibula and tibia are distinct bones in both the
Palæothere and Anoplothere, as in the Pachyderms. It is to the former
genus, however, especially _Pal. magnum_, that the Macrauchene presents
the nearest approach in the general form of the tibia, the principal
bone of its leg: but in the Macrauchene the tibia is relatively shorter,
and thicker, and is straighter and less expanded at its extremities,
especially the upper one, than in any of the Palæotheres.
The mesial boundaries of the two superior articulating surfaces of the
tibia are raised in the form of ridges, which are separated by a deep
groove; of these ridges the external is the highest, as in _Pal.
magnum_: but the articular surfaces in the Macrauchene slope away from
these ridges more than in the Palæotheres. The rotular or anterior
tuberosity of the tibia is more produced, and rises higher than in the
Palæotheres; the ridge continued downwards from this process is more
marked in the Macrauchene, and its limits are better defined: the shaft
of the tibia below the ridge is also more flattened in the
antero-posterior direction than in the Palæothere. The configuration of
the back part of both proximal and distal extremities of the tibia are
so clearly and accurately given in figures 2 and 3, Pl. XIII., as to
render verbal description unnecessary. Neither the text nor the figures
in the ‘Ossemens Fossiles’ afford the means of pursuing the comparison
between the Macrauchene and Palæothere in these particulars; and I
proceed, therefore, to the consideration of the inferior articulating
surface of the bones of the leg (fig. 4, Pl. XIII.)
Since, of the hind-foot, we possess in the present collection only a
single tarsal and metatarsal bone, the structure of the distal articular
surface of the tibia is attended with peculiar interest, because we are
taught by Cuvier that it reveals to us in the Ungulate animals the
didactyle or tridactyle structure of the foot. In the Ruminants this
articular surface is nearly square, and extended transversely between
two perpendicular malleoli, while in the Pachyderms with three toes to
the hind-foot the articular surface of the tibia is oblique, and is
divided into two facets between the perpendicular malleolar boundaries.
Now in the Macrauchenia, although the two bones of the leg are
anchylosed together, the extent of that part of the tarsal articular
surface which is due to the tibia is indicated, as in the case of the
radius in the joint of the fore-arm, by a groove; and we are thus able
to compare this surface with the distal articular surface of the tibia
in the Palæothere and Anoplothere. It presents in the Macrauchenia a
very close resemblance with that of the _Palæotherium magnum_,[25] being
divided into two facets by a convex rising, which traverses the joint
from behind forwards; but the ridge is narrower, the internal facet
somewhat deeper, and the external oblique surface rather flatter than in
the three-toed Palæothere. In the portion of the tarsal articular
surface due to the fibula, we find, however, a more marked deviation
from the Palæothere, and an interesting correspondence with the
Anoplothere, in the inferior truncation and horizontal articular surface
which is continued upon the lower extremity of the fibula, at right
angles with the vertical malleolar facet which forms the outer boundary
of the trochlea of the astragalus: this articular surface unerringly
indicates a corresponding articular projection in the calcaneum, which,
therefore, although the bone itself does not form part of the present
collection, we may conclude to differ from the calcaneum of the
Palæotherium, and to resemble that of the Anoplotherium, in this
particular at least.