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HYRACOIDEA

Online Encyclopedia
Originally appearing in Volume V14, Page 210 of the 1911 Encyclopedia Britannica.
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HYRACOIDEA, a suborder of ungulate mammals represented at the present day only by the Syrian hyrax (Procavia syriaca), the " coney " of the Bible, and its numerous African relatives, all of which may be included in the single genus Procavia (or Hyrax), and consequently in the family Procaviidae. These creatures have no proper English name, and are generally known as hyraxes, from the scientific term (Hyrax) by which they were for many years designated—a term which has unfortunately had to give place to the earlier Procavia. In size these animals may be compared roughly to rabbits and hares; and they have rodent-like habits, hunching up their backs after the fashion of some foreign members of the hare-family, more especially the Liu-Kiu rabbit. In the matter of nomenclature these animals have been singularly unfortunate. In the title " hyrax " they have, for instance, usurped the Greek name for the shrew-mouse; while in the Bible they have been given the old English name for the rabbit. Perhaps rock-rabbit would be the best name. At the Cape they are known to the Dutch as class (badger), which has been anglicized into " dassie." As regards the recent forms, the dentition in the fully adult animal consists only of incisors and cheek-teeth, the formula being i. c. p. #, m. if. There is, however, a minute upper canine developed at first, which is early shed; and in extinct forms this tooth was Fie. i.—The Cape Hyrax (Procavia capensis). functional and molar-like. The tipper 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 anterointernal and a posterior surface, the first two only being covered with enamel; their tips are consequently not chisel-shaped, but sharp-pointed. They are preceded by functional, rooted milk-teeth. The lower incisors have long tapering roots, but not of persistent growth ; and are straight, directed somewhat forwards, with awl-shaped, tri-lobed crowns. Behind the incisors is a considerable gap, followed by the cheek-teeth, which are all contiguous, and formed almost exactly on the pattern of some of the perissodactyle ungulates. The milk-dentition includes three pairs of incisors and one of canines in each jaw. The hyoid arch is unlike that of any known mammai. 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 first is rudimentary, although, in one species at least, all its normal bones are present. The terminal 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 unites 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 a rhinoceros, having three well-developed toes. There is no trace of a first toe, and the fifth meta-tarsal is represented by a small nodule. The terminal phalange 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 pyloric cavity, with a thick, soft, vascular lining. The intestinal canal is long, and has, in addition to the ordinary short, but capacious and sacculated caecum at the commencement of the colon, lower down, a pair of large, conical, pointed caeca. The liver is much subdivided, and there is no gall-bladder. The brain resembles that of typical ungulates far more than that of rodents. The testes are permanently abdominal. The ureters open into the fundus of the bladder as in some Rodents. The Fie. 2.—Skull and Dentition of Tree-Hyrax(Procavia dorsalis) female has six teats, of which four are inguinal and two axillary, and the placenta is zonary and deciduous. There is a gland on the back. The more typical members of the genus are terrestrial in their habits, and their cheek-teeth have nearly the same pattern as in rhinoceroses; while the interval between the upper incisors is less than the width of the teeth ; and the lower incisors are only slightly notched at the cutting edge. Vertebrae: C. 7, D. 22, L. 8, S. 6, C. 6. Of this form the earliest known species, P. capensis, is the type; but there are many other species, as P. syriaca, and P. brucei from Syria and eastern Africa. They inhabit mountainous and rocky regions, and live on the ground. In a second section the molar teeth have the same pattern as in Palaeotherium (except that the third lower molar has but two lobes) ; the interval between the upper incisors exceeds the width of the teeth: and the lower incisors have distinctly trilobed crowns. Vertebrae: C. 7, D. 21, L. 7, S. 5, 1 C. lo. The members of this section frequent the trunks and large branches of trees, sleeping in holes. There are several species from Western and South Africa, as P. arboreus and P. dorsalis. The members of both groups appear to have a power like that possessed by geckos of clinging to vertical surfaces of rocks and trees by the soles of their feet. Extinct Hyracoids.—For many years extinct representative! of the Hyracoidea were unknown, partly owing to the fact that certain fossils were not recognized as really belonging to that group. The longest known of these was originally named Leptodon graecus, but, on account of the preoccupation of the generic title, the designation has been changed to Pliohyrax graecus. This animal, whose remains occur in the Lower Pliocene of both Attica and Samos, wa! about the size of a donkey, and possessed three pairs of upper incisor teeth, of which the innermost were large and trihedral, recalling those of the existing genus. On the other hand, the two outer pairs of incisors were in contact with one another and with the canines, so as to form on each side a series continuous with the cheek-teeth. The next representatives of the group occur in the Upper Eocene beds of the Fayum district of Egypt, where the genera Saghatherium and Megalohyrax occur. These are regarded as representing a distinct family, the Saghatheriidae, characterized by the possession of the full series of twenty-two teeth in the upper jaw, among which the first pair of incisors was modified to form trihedral rootless tusks, while the two remaining pairs were separated from one another and from the teeth in front by gaps. The canine was like a premolar, and in contact with the first tooth of that series; and the cheek-teeth were short-crowned, with the premolar simpler than the molars. and a third lobe to the last lower tooth of the latter series. The members of this genus were small or medium-sized ungulates with single-rooted incisors. On the other hand, the representatives of the contemporary genus Megalohyrax were approximately as large as Pliohyrax, and in some instances had double roots to the second and third incisors. It is now possible to define the suborder Hyracoidea as including ungulates with a centrale in the carpus, plantigrade feet, in which the first and fifth toes are reduced in greater or less degree, and clavicles and a foramen in the lower end of the humerus are absent. The femur has a small third trochanter, the radius and ulna and tibia and fibula are respectively separate, at least in the young, and the fibula articulates with the astragalus. The earlier forms had the full series of 44 teeth, with the premolars simpler than the molars; but in the later types the canines and some of the incisors disappear, and at least the hinder premolars become molar-like. In all cases the first upper incisors are large and rootless. That the group originated in Africa there can be no reasonable doubt; and it is remarkable that so early as the Upper Eocene the types in existence differed comparatively little in structure from the modern forms. In fact the hyraxes were then almost as distinct from other mammals as they are at the present day. See also C. W. Andrews, Descriptive Catalogue of the Tertiary Vertebrala of the Fayum, British Museum (1906). (R. L.*)
End of Article: HYRACOIDEA
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