Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/445

Rh UXGULATA.] MAMMALIA 423 no gall-bladder. The brain resembles that of the typical Ungulates far more than the Rodents. The testes are permanently abdominal. The ureters open into the fundus of the bladder as in some Rodents. The female has six teats, of which four are inguinal and two axillary, and the placenta is zonary, as in the Elephant and Carnivora. There are two distinct forms of Hyrax, differing both in structure .and habits, and which may well be accorded generic rank. 1. Hyrax. Molar teeth having the same pattern as those of Rhinoceros. Interval between upper incisors less than the width of the teeth. Lower incisors slightly notched at the cutting edge. Yertebnc: C 7, U 22, L 8, S 6, C 6. Of this form the earliest known species, //. capensis, is the type. There are several other species, as H. sijriacus and Jutbessinicus, from eastern Africa and Syria. They inhabit mountainous and rocky regions, and live on the ground. 2. Dcndrohyrax. Molar teeth having the same pattern as Palseo- .(Jicriiiiii (except that the third lower molar lias but two lobes). Interval between upper incisors exceeding the width of the teeth. Lower incisors with very distinctly trilobed crowns. Vertebrae : C 7, D 21, L 7, S 5, C 10. The members of this section frequent the trunks and large branches of trees, sleeping in holes. There are several species, not distinctly defined, from western and south Africa, as D. arborcus and D. dorscdis. The members of both groups appear to have a power like that possessed by the Lizards called Geckos of clinging to vertical surfaces of rocks and trees by the .soles of their feet. See HYRAX. The anatomy of Hyrax was first described by Pallas (Spicilegia Zoologica). Besides minor memoirs, two very detailed accounts of its structure have recently appeared one by Brandt, in Mem. Acad. Nat. Scien. St Pe tersbottrg, series vii. vol. xiv., No. 2, 18C9; and another by George, in Annales des Sciences A aturelles, series vi. torn, i., 1874, in which references to all the previous literature will be found. The meehnnism by which the sole of the foot is enabled to adhere to smooth surfaces is fully described by G. E. Dcbson, Proc. Zoo. Soc., 1876, p. 5_ G. SUBORDER PROBOSCIDEA. This name has been appropriated to a well-marked group of animals, presenting so lire very anomalous characters, allied in many respects to the Ungulata, but belonging neither to the Artiodactyle nor Perissodactyle type of that order. It has been thought that they possess some, though certainly not very close, affinities with the Iiodentia, and also with the Sirenia. It is certain, however, that the two species of Elephant which are the sole living representatives of the group, stand quite alone among existing mammals, .differing widely from all others in many points of their .structure. In some respects, as the skull, proboscis, and dentition, they are highly specialized, bub in others, as in the presence of two .anterior venae cavae, and in the struc ture of the limbs, they retain a low or generalized con dition. A considerable series of extinct forms, extending back through the Pliocene and Mioceno epochs, show the same type under different modifications, and in still more .generalized outlines; and certain recently discovered forms from the Eocene of North America, if their affinities are rightly interpreted, appear to link the true Proboscidea to some unknown primitive type of Perissodactyle Un- y id at a. The following are the principal characters common to existing, and, by inference, to the extinct, Proboscidea. The nose extended into a long, muscular, very flexible and prehensile proboscis, at tho end of which the nostrils are situated, and from which the namo given to the group is derived. The teeth consisting of ever-growing incisors of very great size, but never exceeding one pair in each jaw. and often present in one jaw only ; no canines ; large and transversely ridged molars. No clavicles. Limbs strong, the upper segment, especially in tho hind limb, tho longest. Radius and ulna distinct, the latter articulating extensively with the carpus. Fibula and tibia distinct. Astragalus very flat on both surfaces. Manus and pes short, broad, and massive, each with five toes, though the outer pair may be more or less rudimentary, all encased in a common integument, though with distinct, broad, short, hoofs. Third digit the largest. Two anterior venae cavae entering the right auricle. Stomach simple. A capacious caecum. Testes permanently abdominal. Uterus bicor- nuate. Placenta uon-deciduate and zonary. Mammae two pectoral. With regard to the teeth, the incisors, 1 which project largely out of the mouth, and are commonly called &quot; tusks,&quot; are of an elongated conical form, and generally curved. They are composed mainly of solid dentine, the fine elastic quality and large mass of which renders it invaluable as &quot; ivory &quot; for commerce and the arts. A peculiarity of the dentine of the Proboscidea is that it shows, in transverse fractures or sections, striso proceeding in the arc of a circle from the centre to the circumference in opposite directions, and forming by their decussations curvilinear lozenges, as in the &quot;engine-turning&quot; of the case of a watch. The enamel covering in existing species is confined to the extreme apex, and very soon vears ofl, but in some extinct species it forms persistent longitudinal bands of limited breadth. The tusks have small milk predecessors, shed at an early age. The molar teeth present a remarkable series of modifica tions from the comparatively simple form in Dinotherium, with two or three strongly pronounced transverse ridges and a normal mode of succession, to the extremely complex structure and anomalous mode of replacement found in the true Elephants. The intermediate conditions occur in the various species of Mastodon. In this genus the enamel- covered transverse ridges of each tooth are generally more numerous than in Dinotherium, and often complicated by notches dividing their edge or by accessory columns attached IK IV- FIG 102. Longitudinal Sections of the Crown of a Molar Tooth cf various Pro boscideans, showing stages in the gradual modification from the simple to the complex form. I, Mastodon ohioticus ; II, Stegodon insitjnis ; III, Loxodon africanus; IV, Elephas primigenius. The dentine is indicated by transverse lines, the cementum by a dotted surface, and the enamel is black. to them, but in the unworn tooth they stand out freely on the surface of the crown, with deep valleys between (fig. 102, I). In the Elephants the ridges are still further in creased in number, and consequently narrower from before backwards, and are greatly extended in vertical height, so that, in order to give solidity to what would otherwise be a laminated or pectinated tooth, it becomes necessary to envelop and unite the whole in a large mass of cementum, 1 These teeth are by some writers classed as canines, as their roots are implanted in the maxillte ; but, as in Rodents, they are originally developed in the gum covering the premaxilhie, in which bones then- primitive alveoli are sunk. As growth proceeds, however, firm sup port for such massive and weighty bodies can only be obtained by their roots gradually sinking through the premaxilK-e into the great and specially modified alveolar processes of the maxillae, but this does not vitiate their homology with the incisors of other mammals.