268

ONE-TOED LITOPTERNA

CHAP. X

fossa, that it cannot be regarded as anything more than a mark of specialisation. It is, in fact, the case that the Macraucheniidae are in many points specialised, while retaining many primitive features of structure.

The chief primitive features are: the non-alternating positions of the wrist- and ankle-bones; these, of course, interlock in the Perissodactyles of to-day and in many extinct families. Then the absence of a diastema in the tooth series, coupled with the presence in Macrauchenia of a complete dentition. The small brain may be referred to the same category. Macrauchenia must have been a strange-looking animal. It walked upon three toes on each limb; the skull was Horse-like in general form, but the nostrils are removed to a point about as far back as in the Whales or nearly so, the nasal bones being correspondingly reduced. This it is thought argues a proboscis. The humerus is particularly compared by Burmeister' to that of a Horse. The radius and ulna though both well developed are fused. The neck is long, and, as in the Camel, the vertebral arteries run inside the neural arches. Since the fore-legs seem to have been rather longer than the hind-legs, though only very slightly, and the neck was long, the animal may have presented some likeness to the Giraffe. It is interesting to note that in the proportions of humerus to ulna this animal is more Lama-like than Horse-like. On the other hand, the proportions of femur to tibia are more Horse-like. The remains of the creature are limited to South America, and to quite superficial deposits. It is evidently a specialised type, and has pursued a course parallel to that of the Horse. Much nearer to the Horse however, but apparently by convergence only, is the genus Thoatherium, usually placed in a separate family, the Protorotheriidae. In this creature, which has many archaic characters, the toes are reduced to one in each foot. In an allied form, Protorotherium, we have the two lateral toes diminishing just as in Anchitherium.

1 N. Acta Acad. Caes. Leop. Car. xxvii. 1885, p. 238.

CHAPTER XI

UNGULATA (continued)—ARTIODACTYLA (EVEN-TOED

UNGULATES)—SIRENIA

SUB-ORDER 10. ARTIODACTYLA.

THE Artiodactyle or "Even-toed" Ungulates are to be dis

FIG. 138.-Bones of the Manus-A, of Pig (Sus scrofa). . B, of Red Deer (Cervus elaphus). . C, of Camel (Camelus bactrianus). x. X c, Cuneiform; 7, lunar;

m, magnum; m2, m3, second and fifth metacarpals; R, radius; s, scaphoid; td, trapezoid; u, unciform; U, ulna; II-V, second to fifth fingers. (From Flower's Osteology.)

tinguished from the Perissodactyla, and from other Ungulate groups, by a number of trenchant characters. The most salient

270

SOLID-HOOFED PIGS

CHAP.

of these, and that which has given its name to the group, concerns the arrangement of the digits. Instead of there being but one prevailing digit-the third, in the hand and foot, through which the axis of the foot passes, there are two, numbers three and four, between which the same axis passes, and which are perfectly symmetrical with each other. This type of foot has been termed "paraxonic," as opposed to the " mesaxonic" Perissodactyle foot (see Fig. 121 B, p. 235). It has been attempted to prove that the single prevailing digit of the Horse's foot is a fused pair of digits, and the state of affairs which characterises the Camel, where the two metacarpals or metatarsals are to an almost complete extent united, has been urged in proof; so, too, certain abnormalities, such as those called "solid-hoofed pigs." 1 These latter are simply Pigs in which the two central metacarpals and the terminal hoofs are completely fused with one another. In some of such cases there is not the slightest trace of the union of the separate metacarpals and phalanges. Even the sesamoid bones, attached behind to the toes, are two in number instead of four. And, furthermore, the tendon supplying the bones is single, though showing traces of its double origin. Such Pigs often show the abnormality from generation to generation, and they proved convenient for those whose scruples would not allow them to eat the flesh of a beast "dividing the hoof” and not chewing the cud. More singular still, as showing a pathological approach from another side to the Perissodactyle condition in an Artiodactyle, is a calf, where the foot ended in three equi-sized digits, of which the middle one lay in the longitudinal axis of the limb. From the opposite side cases are known of a Horse with a split hoof and phalanges, thus presenting the most striking likeness to a Camel.

There is, furthermore, in certain groups of Artiodactyles (e.g. the Tragulidae) a tendency for the two middle metacarpals to unite, quite apart from such "sports" as those illustrated by the cases just set forth. And, as already mentioned, the union of the two middle metacarpals culminates in the Camel, Ox, etc. There is, however, absolutely no trace of such a fusion in the series of Perissodactyle animals known to us; and it would be by fusion rather than dismemberment that, as it would appear on this theory, the modern Ungulate foot has been arrived at. Of course

1 See Bateson, Materials for the Study of Variation, London, 1894, p. 387.

XI

CARPUS AND TARSUS OF ARTIODACTYLES

271

the facts of Ungulate descent are absolutely destructive of any such comparisons.

As is the case with the Perissodactyles, the Artiodactyles show a historical series, the primitive five-toed condition being almost preserved in Oreodon, up to the most modern modification exemplified by the Ox, Sheep, etc., in which animals there are not even vestiges of the fourth and fifth toes. It has been stated, however, that the foetal Sheep has traces of those rudiments. The so-called cannon bone (the fused third and fourth metapodia) is accompanied in its fusion by an increase in length. At the same time the functional middle metacarpals push aside the rudiments. and, forming a broad surface for that purpose, articulate with the magnum and unciform bones to the exclusion of the rudiments. This has been termed an "adaptive reduction." In the "inadaptive reduction " there is the same reduction of the metacarpals, but the rudiments still articulate as in the primitive Artiodactyle foot, i.e. Mc II with trapezium, trapezoid, and magnum; Mc III with magnum and unciform; Mc IV and V

cb

with unciform. This would appear to give FIG. 139. —Dorsal surface of greater solidity and consequently greater strength to the foot.

right tarsus of Red Deer (Cervus elaphus). × 4. a, Astragalus; c, calcaneum; c, cuneiform; cb, cuboid; mIII, mIV, metatarsals; n, navicular. (From Flower's Osteology.)

In the

The carpal bones of the Artiodactyla alternate in their articulation; the primitive state of affairs is not retained even in the earliest types. The femur has no third trochanter, so prevalent in the Perissodactyles. hind-foot the calcaneum has an articular facet for the fibula, which is not characteristic of the Perissodactyla. In the more modern forms, e.g. the Cervidae, the navicular and cuboid become fused into one bone; and there are even further fusions which will be referred to later as characteristic features of different groups. It is interesting to notice that the reduction begins earlier and is clearer in the hind-foot than in the fore. One

1 See, however, p. 196, for a discussion as to which is the more primitive arrangement.

272

STOMACH OF ARTIODACTYLES

CHAP.

can see how this may be purely adaptive, the push of the hindlegs in running needing a firmer support. In Hyomoschus this is the case. The hind-limbs are provided with a cannon bone, while the metacarpals of the fore-feet are still free.

The number of dorso-lumbar vertebrae is less in the Artiodactyle than in the Perissodactyle Ungulates. Whereas the former have but nineteen, the latter have, as a rule, twenty-three such vertebrae.1 The number of ribs varies from twelve (Camelus, Hydropotes) through thirteen (Cervus, Gazella) to fourteen in Dicotyles, Giraffa, etc.

The curious form of teeth known as " selenodont" is characteristic of the Artiodactyla, though only found well developed in the modern forms, and of those only in the Pecora. The more primitive forms had "bunodont" teeth with typically four tubercles (if we except the tritubercular and but little-known Pantolestes); and the intermediate "buno-selenodont" type characterises such groups as the Anthracotheriidae.

While the stomach of the Perissodactyles is always a simple sac, it is complicated, or shows signs of complication, in the Artiodactyles. That of the Hippopotamus is divided into two chambers; there are three in Tragulus, and four in the typical Ruminants such as Cervus, Ovis, etc.

Had we to deal only with the still living genera of Artiodactyles, it would be easy to sort them into two groups on the characters of the teeth; for the Pigs and Hippopotamus are provided with tubercular molars; they are bunodont. The Deer, Camels, Oxen, Giraffes, etc., have selenodont molars. Besides, the latter are "Ruminants," and have a more complicated stomach. The existing Chevrotains forbid a more trenchant division, since they are, as will be pointed out in due course, somewhat intermediate in structure; the feet are more Pig-like, and the stomach is not so typically Ruminant. In any case such a division is prevented by certain extinct families which are perhaps ancestral to both. They have teeth which are not quite bunodont and not quite selenodont. These teeth have been termed buno-selenodont or buno-lophodont.

The distribution of the living Artiodactyles presents us with some interesting facts. The vast preponderance of species occurs. in the Old World-34 in America as against over 250 species

1 Titanotherium (see p. 266) is exceptional.