The Cambridge Natural History Part 15

ORDER IV. UNGULATA

The existing members of this order can be readily grouped into the Hyracoidea, Proboscidea, Perissodactyla, and Artiodactyla, each of which divisions has quite the value of an order, and all of which are sharply marked off from each other. But as the discovery of so many fossil forms has to a great extent rendered these demarcations less sharp, it is better to regard all these groups as not more than sub-orders of a larger "Order"

Ungulata. Even when this conclusion has been necessarily arrived at from a consideration of the more ancient groups of Ungulate animals, the definition of such an order remains a difficult matter for the systematist.

For the earliest of these forms, more particularly the Ancylopoda, the Amblypoda, and the Condylarthra, whose peculiarities will be dealt with at length subsequently, are not by any means easily differentiated from the primitive Carnivorous mammals of that date, the Creodonta; these latter, moreover, fade into the Marsupials through the so-called Spara.s.sodonta of Professor Ameghino. To confine ourselves to the Ungulates, we may perhaps define them as terrestrial animals with hoofs rather than claws or nails, and chiefly, if not entirely, vegetarian in habit. The teeth are bunodont or lophodont, the tendency to the production of the latter type being always marked. The walk, although plantigrade in the older types, becomes more and more digitigrade, except in such survivals from antiquity as _Hyrax_. There is, too, as we pa.s.s from the ancient types to the modern, a gradual perfection of the limbs as running {196} and not climbing or grasping organs; the number of toes becomes reduced, and culminates twice (in the horse and in the Litopterna) in one toe on each foot; at the same time the ulna becomes rudimentary and fuses with the radius, and the fibula in the hind-limb undergoes a like reduction. The clavicle is absent even in some of the oldest types; its presence in _Typotherium_[120] is highly remarkable. The tail too, an organ which is long in some of the early forms, gets short in their modern derivatives.

[Ill.u.s.tration]

FIG. 110.--An early Ungulate. _Phenacodus primaevus._ 1/12. (After Osborn.)

[Ill.u.s.tration]

FIG. 111.--Series of metacarpals and metatarsals of Camelidae, to show secular and progressive increase in size. From left to right the species are _Protylopus petersoni_, _Poebrotherium l.a.b.i.atum_, _Gomphotherium sternbergi_, _Procamelus occidentalis_. F, Fore-foot; H, hind-foot; III, IV, third and fourth metapodials. (After Wortman.)

Coupled with the increasing perfection of the foot as an organ used merely for the support of the body, certain interesting changes have taken place in the arrangement with regard to each other of the several bonelets of the wrist and ankle. It has been held by Cope and others that the truly primitive disposition of these bones was that presented to us by certain early types, such as _Meniscotherium_ or the existing elephant or _Hyrax_.

In these animals there is (see Fig. 112) a serial {197} arrangement of these bones, the distal bones only, or very nearly only, articulating with the corresponding bones in the upper series. In the modern types (cf. Fig.

113) there is, on the other hand, an interlocking, so that the bones of the distal series articulate with two of those of the proximal series. By this is produced, as it would appear, a much firmer foot, less liable to "give"

under pressure, and thus more fitted for an animal that runs. It is the same principle as that adopted in the laying of bricks. The actual stress and strain of impact has been held responsible for those changes. An equally ingenious and possibly truer explanation of the undoubted facts has lately been advanced by Mr. W. D. {198} Matthew.[121] He has pointed out that in some ancient Ungulates the carpus is not serial but interlocking, even in forms which belong to the earliest Eocene groups, such as the genus _Protolambda_ among the Amblypoda. Now in the fore-foot of _Meniscotherium_ and the living _Hyrax_ there is a separate centrale which is wanting in the greater number of Ungulates. The absorption, that is the practical dropping out of this bone, would restore to an interlocking carpus the serial arrangement; while on the other hand, by the fusion of this bone with the scaphoid, the interlocking disposition would be maintained.

[Ill.u.s.tration]

FIG. 112.--Bones of the ma.n.u.s A, of the Indian Elephant, _Elephas indicus_.

1/8. B, of the Cape Hyrax, _Hyrax capensis_. 1. _c_, Cuneiform; _cc_, centrale; _l_, lunar; _m_, magnum; _p_, pisiform; _R_, radius; _td_, trapezoid; _tm_, trapezium; _s_, scaphoid; _u_, unciform; _U_, ulna. (From Flower's _Osteology_.)

[Ill.u.s.tration]

FIG. 113.--Bones of the ma.n.u.s A, of Rhinoceros, _Rhinoceros sumatrensis_.

1/5. B, of Pig, _Sus scrofa_. 1/3. Letters as in Fig. 112. (From Flower's _Osteology_.)

The gradual perfecting of the fore- and hind-limbs as running organs has been put down to the advent of the gra.s.ses, and the formation of large plains covered with this herbage. The same reason would also be in harmony with the equally gradual change in the shape of the molar teeth, from a tubercular form calculated for a mixed or even a carnivorous diet, to the flatter crushing surfaces exhibited by the lophodont teeth of later Ungulates. Strong {199} canines would in the same way cease to be useful, and even become enc.u.mbrances to such grazing creatures; and their disappearance is one of the salient features in the history of the Ungulata, that is of the modern representatives of the order. The extraordinary hypertrophy of these teeth in such a line as that of the Amblypoda, which has left no descendants, was one of the reasons perhaps for the decay of those great pachyderms of mid-Tertiary times; their excessive armature became an enc.u.mbrance, since it was not accompanied by improvements in other necessary directions. Some of the features of the Tertiary Ungulates have, however, been dealt with in our general sketch of the mammalian life during that epoch, and need not be again referred to here. Of existing Ungulates there are no clear indications of the descent of the Elephants or of the Hyracoidea. Their structure proclaims these two divisions to be of ancient descent, and not to be modern twigs of the Ungulate stem. As to the Perissodactyla and the Artiodactyla we cannot bring them together nearer than in quite early Tertiary times. The order Condylarthra seems to be the starting-point of both these sub-divisions.

_Euprotogonia_ has been considered to be an ancestor of the Perissodactyle branch, and _Protogonodon_ or _Protoselene_ of the Artiodactyla. If this be true, {200} the likenesses which _t.i.tanotherium_ shows to the Artiodactyla must be either purely superficial and secondary, or a cropping out of ancient characters which had been dormant for many generations.

HORNS.--The Ungulata are the only order of mammals which possess horns; as they are on the whole a more defenceless group than the Carnivora, it may be that the horns are a counterpoise to the teeth and claws of the latter; need for defence and for armature in the combats with their own kind for the favours of the does has led to a different kind of protective and aggressive mechanism. Horns as weapons are, however, particularly effective in this group wherever they exist. A Ruminant is most frequently a large and heavy animal without the agility and litheness of the Carnivore. It is precisely to this sort of animal, where weight is an important consideration, that horns are the most suitable weapons. This is further shown by the fact that although the general term horn is used to describe the weapons of the Ungulate mammals, there is more than one kind of structure included under this general term; it is indeed probable that the extreme terms in the series of horns have been independently acquired by their possessors. There is but little in common between the horns of a Giraffe and of a Rhinoceros. In the Rhinoceros we have one or two horns, in the latter case one placed behind the other, which are purely epidermic growths; they may indeed be regarded as matted ma.s.ses of hair, borne, it is true, upon a boss of bone, which however is not a separate structure. The Giraffe supplies us with the simplest term in that series of horns which are partly epidermal and partly bony. The paired horns of this animal have often been contrasted with those of the Deer, for example; but there is no fundamental difference between them. In the Giraffe a pair of bony outgrowths, originally separate from the skull which bears them, but ultimately ankylosed to it, are covered by a layer of entirely unmodified skin. A distinction of undoubtedly practical importance is usually drawn between the Hollow-horned Ruminants, _i.e._ Oxen, Goats and Antelopes, and the Deer tribe. There is nevertheless no fundamental distinction. In the Antelopes there is a core of bone, the "os cornu" as it has been termed, which is covered by a h.o.r.n.y layer, the horn proper, variously modified in shape and size according to the genus or species. In the Deer there is the {201} same os cornu, which may however be branched, but which is in the same way covered by a layer of modified integument; this is known as the "velvet"; it only lasts for a certain period, and is then torn off by the exertions of the animal itself, leaving behind the bony core, which is popularly termed the horn. It will be clear that here is only a difference of comparative unimportance; the same essential features are present in both groups of animals, but the modification of the epidermis has progressed along different lines. Both can be referred back to the primitive conditions seen in the paired horns of the Giraffe. Even the difference, such as it is, is bridged over by the Antelope _Antilocapra_, where the os cornu is bifid and the horn is periodically shed, as is the velvet of the stag; but in the stag the bony part of the horn is also shed, a state of affairs which has no parallel in the Hollow-horned Ruminants.

The great _Sivatherium_ may conceivably be an annectant form between the two types of compound horns, _i.e._ those of the Antelope and those of the Deer. This creature had two pairs of horns, of which, naturally, only the bony cores remain; the hinder pair of these were branched. But although so far they resemble the Deer's horns rather than the Antelope's, Dr. Murie has thought that they were covered by a h.o.r.n.y sheath and not by soft skin as in the Deer. In any case these horns were apparently never shed, which is a point of likeness with the Antelope and of difference from the Deer.

Apart therefore from the nature of the covering of the bony cores, there are good grounds for looking upon them as intermediate between those of the Deer and those of the Antelopes.

The horns of the Ruminants are frequently a secondary s.e.xual character; this is especially the case with the Deer. The Reindeer is, however, an exception, both the stags and the does having horns. That they are a.s.sociated with the reproductive function is shown by their being shed after the period of rut, the destruction of the velvet at that period, and also by the effect upon the horns which any injury to the reproductive glands produces. Some useful facts upon this latter head have been ama.s.sed by Dr. G. H. Fowler,[122] who noticed in a series of stags, horns showing various degrees of degeneration in the antlers produced by varying degrees and periods of gelding. From the facts {202} here collected it is clear that a direct effect is produced. If we are to regard horns as secondary s.e.xual appendages which have been subsequently handed on to the female by heredity, we should expect to meet with examples of animals now horned in both s.e.xes, of which the earlier representatives had the horns confined to one s.e.x. This is most interestingly shown by the extinct and Miocene Giraffe, _Samotherium_, of which the male alone had a pair of short horns, while the skull of the female was entirely hornless; the modern _Giraffa_, as is well known, has horns in both s.e.xes.

It is interesting to note that the existing Perissodactyles and Artiodactyles are to be distinguished by their unpaired or paired horns.

But while there are no Artiodactyles with unpaired horns (save occasional sports) the Perissodactyles have more than once tried, so to speak, paired horns, which ultimately proved fatal to them. The Rhinoceros _Diceratherium_ apparently inherited and improved upon the small paired horns of _Aceratherium_, but it has left no descendant. The paired horned t.i.tanotheria offer another instance of the same apparent incompatibility between the Perissodactyle structure and the persistence of paired horns.

SUB-ORDER 1. CONDYLARTHRA.

This group is characterised by the following a.s.semblage of characters.

Extinct, often plantigrade Ungulates, with five-toed limbs. Bones of carpus and tarsus not always interlocking, but sometimes lying above each other in corresponding positions. The humerus has an entepicondylar foramen. Dental formula quite complete; the molars brachyodont and bunodont. The premolars are simpler than the molars. The canines are small. As with other early types, the zygapophyses are flat and do not interlock. The astragalus is like that of the Creodonta. This group was American and European in range, the remains of its rather numerous genera being of Eocene time. The best-known genus is _Phenacodus_, of which some account will be given before discussing the, in many cases, more fragmentary remains of other allied forms.

The genus _Phenacodus_ was first described so long ago as 1872, from a few scattered teeth. Since then several nearly complete skeletons have been obtained, and we are in full possession of {203} the details of its osteology. It was not a large creature (see Fig. 110, p. 196), about 6 feet in length, with a small head. The feet were more or less plantigrade, and five-toed. The last phalanges of the toes show that they carried hoofs and not claws; yet the fore-feet look a little as if they could be used as grasping organs. The third digit of both hind- and fore-feet exceeds the others, and thus a Perissodactyle-like foot characterised this Eocene creature. The tail is exceedingly long, and must have reached the ground as the animal walked. This is of course by no means an Ungulate character.

Still, in the totality of its organisation the animal was decidedly Ungulate, though Professor Cope spoke of _Phenacodus_ as not merely an ancestral Ungulate but as the parent form of Insectivores, Carnivores, Lemurs, Monkeys, and Man himself! The scapula indeed is from its breadth and oval contour rather like that of a Carnivore. The clavicles as in other Ungulates are absent. The femur is Perissodactyle rather than Artiodactyle in the presence of a third trochanter. The creature had fifteen pairs of ribs and five or six lumbar vertebrae. The two bones of the leg which lie below the femur are perfectly distinct and separate. A cast of the brain-case shows that the cerebral hemispheres were smooth and small, the cerebellum of course completely uncovered and nearly as large as the cerebrum. The olfactory lobes were also large. The complete skeleton of _Phenacodus_ has lately been excavated more fully from the enveloping matrix by Professor Osborn,[123] and mounted in what is regarded as the natural position of the beast. It appears that though five-toed it went upon the three middle toes only, and furthermore that of these the middle one was the more prevailing, so that _Phenacodus_ was distinctly "Perissodactyle," at least in habit. Moreover its "long hind-quarters, the long powerful tail ... are reminiscent of Creodont ancestry." The genus was European and American in range.

_Meniscotherium_ ( = _Hyracops_[124]) comprises several forms of about the size of a fox; they are both European and American in range. The teeth are more distinctly Ungulate in form than those of _Phenacodus_, with a [125]-shaped outer wall. The skull is described as possessing "indifferent, primitive characters," permitting a comparison with those of Opossums, Insectivores, and {204} Creodonta. It has, as in _Phenacodus_, no orbital ring. The humerus resembles that of a Carnivore rather than that of an Ungulate. The carpus and tarsus are serial. The fibula articulates with both the calcaneum and the astragalus, which is not the case with _Phenacodus_. It is suggested that these animals are ancestral forms of the Chalicotheres. In the brain the hemispheres do not cover the cerebellum.

More primitive apparently than _Phenacodus_ was the less-known genus _Euprotogonia_, or _Protogonia_[126] as it has been called. The best-known species is _E. puercensis_, so called from its occurrence in the Puerco beds of the American Eocene. It was a slender, long-limbed creature, smaller than _Phenacodus_, with a long and heavy tail as in that animal.

Like _Phenacodus_ it was semiplantigrade, and shows more likenesses to the Creodonta. The skull is only known by a part of the lower jaw with teeth, and by the teeth of the upper jaw. The vertebrae are not entirely preserved, but enough remain to show that the animal had a tail of 16 or 17 inches, which is a considerable length when compared to its height, about a foot at the rump. In the fore-limb the most noteworthy point is that the ulna has a convex posterior border as in the Creodonts, the same border in _Phenacodus_ being concave. The humerus is slender, with less-marked tuberosities. The fifth digit seems to have been less reduced. The phalanges seem to have borne h.o.r.n.y sheaths somewhat intermediate between hoofs and claws. The pelvis is described as being, as is also that of _Phenacodus_, rather like that of the Creodonta. The right hind-limb is known in all its details. It appears that the bones are not serial but interlocking; this, however, on the views with regard to the relations of these two forms of tarsus mentioned on p. 198, does not militate against regarding _Euprotogonia_ as the ancestor of the genus _Phenacodus_. The third toe is the pre-eminent one, the animal thus being Perissodactyle. The lateral digits are larger than in _Phenacodus_, and the metatarsals and the phalanges are slightly curved, which is again a Creodont character as compared to the perfectly straight corresponding bones of _Phenacodus_. It seems evident that this animal is to be looked upon as a more ancient type than _Phenacodus_, even if not as its actual ancestor.

Another group of the Condylarthra contains the genus _Pertipychus_ and some others. _Periptychus_ has the full dent.i.tion {205} of forty-four teeth, the molars being of course bunodont, with the three chief tubercles most developed. The bones of the tarsus interlock and are not serial, as they are in many other members of the Condylarthra. The astragalus has a shorter neck than in _Meniscotherium_, for example. It has in this a likeness to the same bone in the Amblypoda, to the primitive members of which, such as _Pantolambda_, this animal bears much resemblance. "Astragali and many skeletal bones of _Periptychus rhabdodon_ and _Pantolambda bathmodon_ are almost indistinguishable," observes Mr. Matthew. The fore-feet of this genus are unknown, but it would seem that it was plantigrade from the evidence of the hind-feet. There are several species of the genus.

Possibly, but not at all certainly, the Mioclaenidae, with the genera _Mioclaenus_ and _Protoselene_, are to be referred to this same order of primitive Ungulates. It is only necessary to mention them here, because they show very clearly the primitive form of dent.i.tion of these early Eocene mammals. The teeth are quite complete and unbroken by a diastema.

The canines are but little p.r.o.nounced. The molars are not strictly tritubercular, but have a prevailing trituberculy. The nature of the feet is not known. Since the genus _Protoselene_, as its name denotes, shows an indication of a commencing selenodonty, it has been suggested that this group is the stock whence the Artiodactyles have been derived.

In any case, whether the particular comparisons that have been made as to the relationship of various forms of Condylarthra are valid or not, it seems to be plain that this group represents the earliest Ungulate stock, but little differentiated from the contemporaneous Creodonts.

SUB-ORDER 2. AMBLYPODA.

This group of extinct mammals has the following princ.i.p.al characteristics:--

They are large, semiplantigrade Ungulates, of heavy build and apparently elephantine gait. The dent.i.tion is for the most part complete as in other ancient groups, and the canines are in the later forms big tusks. The back teeth are brachyodont and ridged (lophodont). Both radius and ulna in the fore-limb, and tibia and fibula in the hind-limb, are well developed. The bones {206} in the carpus are alternating in position. The toes are five in both feet, and are very short. There is a hint of commencing "perissodactylism" in the fore-feet at any rate. The brain is small and the hemispheres smooth.

The Amblypoda, or Amblydactyla, are so called on account of their short and stumpy feet and toes. They were held by Professor Cope to be on the direct line of ancestry of both Perissodactyles and Artiodactyles, a view which is on the whole not accepted at present.

[Ill.u.s.tration]

FIG. 114.--Skull of _Protolambda bathmodon_. . _e.a.m_, External auditory meatus; _m_, mastoid; _m.f_, mastoid foramen. (After Osborn.)

As is the case with other groups, the Amblypoda commenced existence as a sub-order with relatively small forms such as _Pantolambda_, the most ancient type known, which is in many respects a transition between the later forms and other groups of mammals such as the Creodonta.[127] The race culminated and ended in the giant _Dinoceras_ and _Coryphodon_, and spread into the Old World. In spite of their smooth and diminutive brain, these mammals were able to hold their own and to multiply into many species and genera; in this they were perhaps aided by their formidable tusks and by the horns which many of them possessed. The teeth seem to imply an omnivorous diet, which was quite possibly an additional advantage in the struggle for existence. It does not seem to be necessary to divide off the Dinoceratidae into a sub-order equivalent to the Coryphodontidae as was done {207} by Professor Marsh; the numerous points in common possessed by the members of both families forbid their separation more widely than as families.

The earliest types of Amblypoda belong to the genus _Pantolambda_, of which the species _P. bathmodon_ was about four feet in length. As restored it seems to have had proportionately short fore- and hind-limbs, and it had a long tail. It was apparently plantigrade, and would have had not a little likeness to a carnivorous type. The skull has no air cavities, such as are developed in the later types from the Lower Eocene, e.g. _Coryphodon_; _Pantolambda_ is from the basal Eocene. The frontal bones show no trace of the horns that are developed in subsequent forms; the nasals are comparatively long; the zygomatic arch is slender. The molar teeth are in the primitive form of trituberculy, and the premolars, as is so often the case with primitive animals, are unlike the molars in form, being less markedly selenodont. As to the vertebral column, the dorsal vertebrae appear to have had short spines, which argues, as it does also in the case of the larger and heavier _Coryphodon_, a feebleness in the development of ligaments and muscles supporting and moving the head. The scapula seems to have the same peculiar leaf-like form that it has in the later _Coryphodon_.[128] This primitive type shows an entepicondylar foramen in the humerus. It is interesting to observe that the posterior border of the ulna is convex, as in the Creodonts, and in the early Condylarthrous form _Euprotogonia_. In the subsequently-developed Amblypoda, as in the later Condylarthra, that bone acquires a concave outer border. In the carpus the os centrale is distinct. In the femur the third trochanter is well formed; it gradually dies out in later Amblypoda. The fibula articulates with the calcaneum. This species, according to Osborn, "typifies the hypothetical Protungulate, being more primitive than either _Euprotogonia_ or _Phenacodus_."[129]

[Ill.u.s.tration]

FIG. 115.--Skeleton of _Coryphodon radians_. 1/10. (After Osborn.)

The genus _Coryphodon_ is known by a large number of species, of which the first was discovered in this country, and was represented merely by a jaw with some teeth. This was named by Sir R. Owen _C. eocaenus_, and was dredged up from the bottom of the sea off the Ess.e.x coast. A second specimen consisted of a single {209} canine tooth only, and was brought up from a depth of 160 feet during the making of a well at Camberwell. More abundant remains have since been found in North America.

This genus had a large head, and in some specimens traces of the "horn cores," so marked in the related _Dinoceras_, are to be noticed. The skull is broad behind and narrowed in front; the roofing bones show the cellular s.p.a.ces so characteristic of the Elephant. The jugal bone, however, is not, as it is in the Elephant, placed in the middle of the somewhat ma.s.sive zygomatic arch. As in some other primitive Ungulates (e.g. _Phenacodus_) there are twenty dorso-lumbar vertebrae, of which fifteen bore ribs.

The scapula seems to have possessed a peculiar leaf-like form, swelling in the middle and ending almost in a point above. It has a well-marked spine, and the acromion projects much. The fore- as well as the hind-feet are in a state of transition between plantigradism and digitigradism. It was at one time held that the animal was digitigrade as to the fore-feet and plantigrade as to the hind-feet. Though, as has been pointed out, it is a fact that the hind-feet are often on a different plane of evolution from the fore-feet, it seems that this amount of difference does not characterise any Ungulate, not excepting the genus now under consideration.

The toes are very spreading. The pelvic girdle is of great strength and broadness. The femur, as in the Perissodactyles, has a well-developed third trochanter; but whereas in this particular the hind-limb is Perissodactyle, it is Artiodactyle in the fact that the tibia and the fibula articulate with the astragalus and calcaneum. The ridged teeth have given the name to the genus.

A curious feature in the structure of the genus are the slender spines of the dorsal vertebrae, which contrast with the enormous ones of some other Ungulates--more curious in this genus, which is of heavy build, than in the lighter _Pantolambda_. The back of the animal is short, and the limbs are very spreading, so that the gait was doubtless shuffling. The large head, and short and heavy limbs and limb girdles added probably to its c.u.mbrous walk or trot. The canines are great tusks, and spread out on both sides of the mouth.[130]

The late Professor Cope, in 1874, described the probable appearance of the _Coryphodon_ in the following words:--"The general appearance of the Coryphodons, as determined by the skeleton {210} probably resembled the bears more than any living animals, with the important exception that in their feet they were much like the elephant. To the general proportions of the bears must be added the tail of medium length. Whether they were covered with hair or not is of course uncertain. Of their nearest living allies, the elephants, some were hairy and others naked.... The movements of the Coryphodons doubtless resembled those of the elephant in its shuffling and ambling gait, and may have been even more awkward from the inflexibility of the ankle."

The most recent members of this sub-order come from the Middle Eocene beds, and are chiefly referable to the genus _Dinoceras_, with which _Tinoceras_ and _Uintatherium_ are at least very nearly related, if not identical.

These creatures were of great size, larger than the earlier types which have been considered. They show a certain superficial resemblance to the t.i.tanotheriidae, on account of the ma.s.sive horn cores upon the skull. These horn cores are large upon the maxillae and the parietals, and are paired; on the nasals are smaller horns. The bones of the skull have air cavities.

The incisors of the upper jaw are absent; the canines are enormous tusks, and the lower jaws are f.l.a.n.g.ed downwards near the symphysis where these tusks border them. Contrary to what is found in the older types, where the position of the condyle of the lower jaw is normal, this prominence faces backwards in the Dinocerata. The same shortness of the spines of the dorsal vertebrae prevails in this group as in the other Amblypoda, though it is perhaps hardly so marked. The scapula has not the peculiar ac.u.minate form that exists in _Coryphodon_, but is triangular and broad above. The limbs are elephantine, in that the angle between the humerus and the femur respectively, and the bones which follow, is not marked. The hind-limbs are especially straight. The tail is short as compared with that of the primitive Amblypoda. The Dinocerata are purely digitigrade. The entepicondylar foramen has, as in the Coryphodonts, disappeared. The os centrale of the carpus has become fused, and no longer exists as a separate bone. The fibula no longer articulates with the calcaneum, but both that bone and the ulna are well developed. The genus _Astrapotherium_ is placed among the Amblypoda by some authorities.[131]

{211}

SUB-ORDER 3. ANCYLOPODA.