The Hyracoidea

The genus Hyrax, which is the sole member of this group, was referred by Pallas to the Rodents; and by Cuvier, who demonstrated that it could not be a Rodent, it was placed among the Ungulata, in the immediate neighborhood of Rhinoceros, without any better evidence than that afforded by the characters of the molar teeth. Prof. Brandt, of St. Petersburg, in an elaborate memoir just published, arrives at the conclusion that it is a "gliriform Ungulate," intermediate, in a certain sense, between the Rodents and the Ungulata; but, still, more Ungulate than Rodent. It appears to me to be neither Ungulate nor Rodent, but the type of a distinct order, in many respects intermediate between the Ungulata, on the one hand, and Rodentia and Insectivora, on the other.

The small, Rabbit-like, animals comprised in the genus Hyrax are plantigrade, and provided with four visible toes in front and three behind. The nails are not hoof-like, but nearly flat, except the innermost of the biud-foot, which is peculiarly curved. The body is covered with fur, and the muffle, or snout, is split, as in the Rodents. There is a pendulous penis, but no scrotum; and there are four inguinal and two axillary teats.

There are from twenty-nine to thirty-one dorso-lumbar vertebrae, which is the greatest number known in any terrestrial mammal. Twenty-one or twenty-two of these are dorsal. No mammal, except Choloepus, the two-toed Sloth, possesses so large a number of dorsal vertebrae as this. The transverse processes of the last lumbar vertebra articulate with the sacrum, as is the case in many Ungulate Mammals. In the skull, the post-orbital processes, which are chiefly furnished by the parietal and the jugal, nearly meet. Part of the articular facet for the mandible is formed by the jugal, which extends forward until it comes into contact with the lachrymal bone. The base of the external pterygoid process is perforated by a canal, as in Perissodactyla and Lemuridae. There are large pre-and post-tympanic processes, and the post-tympanic is much shorter than the par-occipital process. The premaxillae are large, and unite extensively with the nasal bones; the perpendicular ramus of the mandible is very wide, and somewhat like that of the Tapir in shape. The posterior margin of the bony palate is opposite the anterior edge of the last molar tooth.

The scapula is devoid of an acromion process, as in the Perissodactyla. There are no clavicles, but the coracoid process is well developed. The ulna is complete, and a rudiment of the pollex is present. In the carpus, a line prolonging the axis of the third metacarpal bisects the os magnum and the lunare, which is not the case in any Ungulate Mammal.

In the hind-limb, the femur possesses a small third trochanter, which is not nearly so conspicuous as in some Rodents. The tibia and fibula are complete. The extremity of the inner malleolus articulates with a shelf-like process of the astragalus, the distal face of which bone has no facet for the cuboid. The digits i. and v. are not represented even by rudiments. The terminal phalanx of ii. is longitudinally cleft.

The dentition of the adult is i. 2.2/2.2 c. 0.0/0.0 p.m. 4.4/4.4, and m. 3.3/3.3. The outer upper incisors are very small, and soon fall out; the inner, which are very large, curved, and have a thick coat of enamel on their anterior faces, continue to grow throughout life, as in Rodents. The lower incisors have crowns denticulated at the edges, like those of Galeopithecus and some Bats. They bite upon a callous pad which lies behind the upper incisors. The patterns of the upper and lower molar teeth are very similar to those of the corresponding teeth in Rhinoceros. As in the Horse, part of the Eustachian tube is dilated into a thin-walled sac extending on the inner side of the bulla tympani from the pterygoid processes to the exit of the ninth nerve.

A slight constriction marks oiF the cardiac from the pyloric division of the stomach. The cardiac portion is lined by a dense epithelium. The intestine is provided with three caeca-one in the ordinary position, and two placed much lower down on the colon, opposite one another, and terminating by pointed ends. There is no gall-bladder. The ureters open, not near the neck of the bladder as in Mammals generally, but near the fundus, as in some few Rodents.

The male has vesiculae seminales, prostatic and Cowperian glands. The uterus is two horned, and the vulva and anus are surrounded by a common fold of integument.

In the foetus the yelk-sac and the vitello-intestinal duct early disappear. The amnion is not vascular. The allantois spreads over the interior of the chorion, and gives rise to the broad zone-like placenta, which is composed of both maternal and foetal parts. The maternal vessels pass straight through the thickness of the placenta toward its foetal surface, on which they anastomose, forming meshes, through which the vessels of the foetus pass toward the uterine surface of the placenta.

The species of the genus Hyrax are found only in Syria and Africa. No fossil Hyraecoidea are known.

The Discoidea. - The Mammalia with discoidal placentae are the Rodentia, the Cheiroptera, the Insectivora, and the Primates.