Page:EB1911 - Volume 21.djvu/186

Rh PERIPTERAL (Gr., round, and , a wing), in architecture, the term applied to a temple or other structure where the columns of the front portico are returned along its sides as wings at the distance of one or two intercolumniations from the walls of the naos or cella. Almost all the Greek temples were peripteral, whether Doric, Ionic, or Corinthian (see ).  PERISSODACTYLA (i.e. odd-toed), the name proposed by Sir R. Owen for that division of ungulate mammals in which the toe corresponding to the middle (third) digit of the human hand and foot is symmetrical in itself, and larger than those on either side (when such are present). The Perissodactyla have been brigaded with the (q.v.) to form the typical group of the ungulates, under the name of Diplarthra, or Ungulata Vera, and the features distinguishing the combined group from the less specialized members of the order Ungulata will be found under the heading of that order.

The following are the leading characteristics by means of which the sub-order Perissodactyla is distinguished from the Artiodactyla. The cheek-teeth (premolars and molars) form a continuous series, with massive, quadrate, transversely ridged or complex crowns—the posterior premolars usually resembling the molars in structure. Crown of the last lower molar commonly bilobed. Dorso lumbar vertebrae never fewer than twenty-two, usually twenty-three in the existing species. Nasal bones expanded posteriorly. An alisphenoid canal. Femur with a third trochanter. The middle or third digit on both fore and hind feet larger than any of the others, and symmetrical in itself, the free border of the terminal phalanx being evenly rounded (see fig. 1) This may be the only functional toe, or the second and fourth may be subequally developed on each side. In the tapirs and many extinct forms the fifth toe also remains on the fore-limb, but its presence does not interfere with the symmetrical arrangement of the remainder of the foot on each side of the median line of the third or middle digit The astragalus has a pulley-like surface above for articulation into the tibia, but its lower surface is flattened and unites to a much greater extent with the navicular than with the cuboid, which bone is of comparatively less importance than in the Artiodactyles. In existing forms the calcaneum does not articulate with the lower end of the fibula. The stomach is simple, the caecum large and capacious, the placenta diffused, and the teats inguinal.

The Perissodactyla may be divided into the four following sections, namely the extinct Titanotheroidea, the Hippoidea, represented by the horse tribe and their ancestors, the Tapiroidea,

typified by the tapirs, and the Rhinocerotoidea, which includes the modern rhinoceroses and their forerunners.

1. Titanotheres.—In the Titanotheroidea the dentition may be expressed by the formula i, c, p, m. There is usually a short gap between the canine and first premolar; the upper molars are short-crowned and transitional between the bunodont (tubercular) and selenodont (crescentic) types, with two outer concave tubercles and two inner conical ones; while the lower molars are crescentic, with three lobes in the last of the series. The skull is elongated, with the orbit not separated from the temporal fossa and the nasals, which may or may not carry horns, reaching at least as far forwards as the union of the premaxillae. The post-glenoid, post-tympanic and paroccipital processes of the skull are large, and there is an alisphenoid canal. There are four functional toes in front and three behind; while the calcaneum, unlike that of the other three groups, articulates with the fibula. The group is represented by the families Palaeosyopidae and Titanotheriidea in the Tertiary deposits of North America. Both families are described under the heading.

2. Horse Group.—In the Hippoidea there is generally the full series of 44 teeth, but the first premolar, which is always small, is often deciduous or even absent in the lower or in both jaws. The incisors are chisel-shaped, and the canines tend to become isolated, so as in the more specialized forms to occupy a more or less midway position in a longer or shorter gap between the incisors and premolars. In the upper molars the two outer columns or tubercles of the primitive tubercular molar coalesce to form an outer wall, from which proceed two crescentic transverse crests, the connexion between the crests and the wall being slight or imperfect, and the crests themselves sometimes tubercular. Each of the lower molars carries two crescentic ridges. In the earlier forms the cheek-teeth are low-crowned, but in the higher types they become high-crowned. The number of front toes ranges from four to one, and of hind ones from three to one. The post-glenoid, post-tympanic and paroccipital processes of the skull are large; the second of these being always distinct. Nasals long, normally without traces of horns.

The section is divisible into the families Equidae and Palaeotheridae, of which the latter is extinct.

In the Equidae the premolars are generally or. In the earlier short-crowned forms these teeth are unlike the molars, and the first of the series is separated by a gap from the second. In the high-crowned types, as well as in some of the intermediate ones, they become molar-like, and roots are not developed in the whole cheek-series till late. Orbit in higher forms closed by bone; and ridges of lower cheek-teeth terminating in large loops. Front toes 4, 3 or 1, hind; 3 or 1. (See and .)

In the Palaeotheriidae the premolars may be or, and are generally molar-like, while the first (when present) is always close to the second, all the cheek-teeth short-crowned and rooted, with or without cement. Outer walls of upper cheek-teeth W-shaped, and transverse crests oblique. Orbit open behind; and ridges of lower cheek-teeth generally terminating in small loops. Feet always 3-toed. (See .)

3. Tapir Group.—In the Tapiroidea the dentition may be either the full 44, or lack the first premolar in the lower or in both jaws. The incisors are chisel-shaped; and (unlike the early Hippoidea) there is no gap between the first premolar, when present, and the second. The upper cheek-teeth are short-crowned and without cement, and show distinct traces of the primitive tubercles; the two outer columns form a more or less complete external wall, connected with the inner ones by a pair of nearly straight transverse crests; and the premolars are originally simpler than the molars. Lower cheek-teeth with two straight transverse ridges. Nasals long in early, but shorter in later forms, hornless; orbit open behind. Front toes, 4; hind toes, 3.

This group is also divided into two families, the Tapiridae and Lophiodontidae, the latter extinct.

In the Tapiridae the dentition may be reduced below the typical 44 by the loss of the first lower premolar. Hinder premolars either simple or molar like. Outer columns of upper molars similar, the hinder ones not flattened; ridges of lower molars oblique or directly transverse, a third ridge to the last molar in the earlier forms. The Lophiodontidae, which date from the Eocene, come very close to Hyracotherium in the horse-line; and it is solely on the authority of American palaeontologists that the division of these early forms into equoids and tapiroids is attempted. In North America the earliest representative of the group is Systemodon of the Lower Eocene, in which all the upper premolars are quite simple; while the molars are of a type which would readily develop into that of the modern tapirs, both outer columns being conical and of equal size. The absence of a gap between the lower canine and first premolar and between the latter and the following tooth is regarded as an essentially tapir-like feature Lophiodochoerus apparently represents this stage in the European Lower Eocene; Isectolophus, of the American Middle Eocene, represents a distinct advance, the last upper premolar becoming molar-like, while a second species from the Upper Eocene is still more advanced; the third lobe is, however, retained in the last lower molar. In the