Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/444

Rh 422 MAMMALIA [UNGULATA. and yet are evidently allied to both, and which to a certain extent bridge over the interval between these and the isolated groups just mentioned, makes it necessary either to introduce a number of new and ill-defined ordinal divisions, or to widen the scops of the original order so as to embrace them all. They are all animals eminently adapted for a terrestrial life, and in the main for a vegetable diet. Tlfough a few are more or less omnivorous, and may under some cir cumstances kill living creatures smaller and weaker than themselves for food, none are distinctly and habitually predaceous. Their teeth are markedly heterodont and diphyodont, the milk set being well developed and not completely changed until the animal attains its full stature. The molars have broad crowns with tuberculated or ridged surfaces. They have no clavicles. Their toes are provided with blunt, broad nails, or in the majority of cases with hoofs, more or less enclosing the ungual phalanges. The scaphoid and lunar bones of the carpus are always distinct. The whole group may be divided into the Ungulata Vera, containing the suborders Perissodactyla and Artiodactyla, and a less well-known assemblage of animals which may be called Subungulata or Ungulata Polydactyla. Cope lias pointed out a character in the struc ture of the carpus by which the latter are dif ferentiated from the for mer. In all the Subungtt- lata the bones of the proximal and distal row retain the primitive or more typical relation to each other- (see fig. 100). The os magnum of the second row articulates mainly with the lunar IT of the first, or with the cuneiform, but not with the scaphoid, while in the group to which the vast majority of modern FlG loo.-Right Fore Foot of Indian Eie- belon^ the P hant - *|. U, ulna; R, radius ;&amp;lt;, cuneiform; ,. . !, /, lunar ; sc, scaphoid; u, unciform; m, mag- Or distal row has num; td, trapezoid ; tm, trapezium ; I to V, been shifted altogether ^ to fifth digit, towards the inner side of the limb (see figs. 107 and 109), so that the magnum is brought considerably in relation with the scaphoid, and is entirely removed from the cunei form; as in the great majority of existing mammals. SUBUNGULATA. By far the greater number of the Subunguhvta are extinct, and of many of those whose former existence has been revealed, chiefly by the labours of the American palaeonto logists, our knowledge is at present necessarily imperfect, though daily extending. It will only be possible here to give any details of some of the more interesting or best- It txown forms. SUBORDER IIYRAC01DEA. This division is constituted to receive a single family of mammals, the affinities of which have long constituted a puzzle to zoologists. They were first placed among the Rodents, to which animals their small size and general appearance and habits give them much superficial resem blance. Cuvier s investigations into their anatomical structure, and especially their dental characters, led him to place them among the Ungulates, near the genus Rhinoceros, a position still accepted by many zoologists. Further knowledge of their organization and mode of development has caused Milne-Edwards, Huxley, and others to disassociate them from this connexion, and, failing to find any agreement with any other known forms, to place them in a group entirely apart. Palaeontology has thrown no light upon the affinities of this anomalous and isolated group, as no extinct animals possessing their distinctive characters have as yet been discovered. The dentition consists only of incisors and molars, the formula in all known species being i, c $, p -*, m. The upper incisors have persistent pulps, and are curved longitudinally, forming a semicircle as in Rodents. They are, however, not flattened from before backwards as in that order, but prismatic, with an antero-external, an antero-internal, and a posterior surface, the first two only being covered with enamel ; their apices are consequently not chisel-shaped, but sharp pointed. They are preceded by functional, rooted milk teeth. The lower incisors have FIG. 101. Skull and Dentition of Dendrohyrax dorsalis. x f. long tapering roots, but not of persistent growth. They are straight, procumbent, with awl-shaped, trilobed crowns. Behind the incisors is a considerable diastema or interval. The molars and premolara are all contiguous, and formed almost exactly on the pattern of some of the Perissodactyle Ungulates. The hyoid arch is unlike that of any known mammal. The dorsal and lumbar vertebrae are very numerous, 28 to 30, of which 21 or 22 bear ribs. The tail is extremely short. There are no clavicles. In the fore foot, the three middle toes are subequally developed, the fifth is present, but smaller, and the hallux is rudimen tary, although, in one species at least, all its normal bones are present. The ungual phalanges of the four outer digits are small, somewhat conical, and flattened in form. The carpus has a distinct os centrale. There is a slight ridge on the femur in the place of a third trochanter. The fibula is complete, thickest at its upper end, where it generally ankyloses with the tibia. The articulation between the tibia and astragalus is more complex than in other mammals, the end of the malleolus entering into it. The hind foot is very like that of Rhinoceros, having three well-developed toes. There is no trace of a hallux, and the fifth metatarsal is represented by a small nodule only. The ungual phalanx of the inner (or second) digit is deeply cleft, and has a peculiar long curved claw, the others having short broad nails. The stomach is formed upon much the same principle as that of the Horse or Rhinoceros, but is more elongated transversely and divided by a constriction into two cavities a large left cul de sac, lined by a very dense white epithelium, and a right pylorio cavity, with a very thick, soft, vascular lining. The intestinal canal is long, and has an arrangement perfectly unique among mammals, indeed among vertebrated animals, for, in addition to the ordinary short, but capacious and sacculated caecum at the commencement of the colon, there is, lower down, an additional pair of large, conical, pointed caeca. The liver is much subdivided, and there is