UNGULATA.] MAMMALIA 431 The Pecora or true Ruminants form at the present time an ex tremely homogeneous group, one of the best-defined and most closely united of any of the Mammalia. But, though the original or com mon type has never been departed from in essentials, variation has been very active among them within certain limits ; and the great difficulty which all zoologists have felt in subdividing them into natural minor groups arises from the fact that the changes in dif ferent organs (feet, skull, frontal appendages, teeth, cutaneous glands, &c. ) have proceeded with such apparent irregularity and absence of correlation that the different modifications of these parts are most variously combined in different members of the group. It appears, however, extremely probable that they soon branched into two main types, represented in the present day by the Cerridee and the Bovidx, otherwise the Antlered and Horned Ruminants. In termediate smaller branches produced the existing Musk-Deer and Giraffe, as well as the extinct Hell adothcrium inclining to the first- named group and the extinct Sirathcrium, Brahmatherium, //?/- dnspitherium and others more allied to the latter, although upon the true relationship of these forms there is a difference of opinion between the two palaeontologists who have paid most attention to the group, Riitimeyer and Lydekker, but the materials forthcoming at present are scarcely sufficient for forming a decided opinion. The earliest forms of true Pecora, as Gelocus and Drcmothcrium (Miocene), had no frontal appendages, and some few forms (Moschus and Hydropotes) continue to the present day in a similar case. In the very large majority, however, either in both sexes or in the male only, a pair or occasionally two pairs (Tctraccros and the extinct Sivathcrium) of processes are developed as weapons of offence and defence from the frontal bones, these being almost always formed on one or other of two types. 1. "Antlers" are an outgrowth of true bone, covered during their growth with vascular, sensitive integument coated with short hair. In this state they remain permanently in the Giraffe, but in the true Ccrvidee, when the growth of the antler is complete, the supply of blood to it ceases, the skin dies and peels off, leaving the bone bare and insensible, and after a time, by a process of absorption near the base it becomes detached from the skull and is "shed." A more or less elongated portion or " pedicle " always remains on the skull, from the summit of which A fi FIG. 112. Head of Deer (Cervus schomburgl-ii), showing Antlers. From Sclater, 1 roc. Zool. Soc., 1877, p. 682. a new antler is developed. In the greater number of existing species of Deer this process is repeated with great regularity at the same period of each year. The antler may be simple, straight, subcylmdrical, tapering and pointed, but more often it sends off one or more branches called " tynes " or " snags." In this case the main stem is termed the "beam." Commonly all the branches of the antler are cylindrical and gradually tapering. Sometimes they aro more or less expanded and flattened, the antler being then said to be "palmated." In young animals the antlers are always small and simple, and in those species in which they are variously branched or palmated, this condition is only gradually acquired in several successive annual growths. An interesting parallel has been observed here, as in so many other cases, between the development of the race and that of the individual. The earliest known forms of Deer, those of the Lower Miocene, have no antlers, as in the young of the existing species. The Deer of the Middle Miocene have simple antlers, with not more than two branches, as in existing Deer of the second year. Species occur in the Upper Miocene with three branches to the antlers, but it is not until the Upper Pliocene and Pleistocene times that Deer occur with antlers developed with that luxuriance of growth and beauty of form characteristic of some of the existing species in a perfectly adult state. Among recent Cervidie, antlers are wanting in the genera Moschus and Hydropotes ; they are present in both sexes in Tarandus (the Reindeer), and in the male sex only in all others. 2. The horns " of the Bovidee, consist of permanent, conical, usu ally curved, bony processes, into which air-cells continued from the frontal sinuses often extend, called "horn-cores," ensheathed in a case of true horn, an epidermic development of fibrous structure, which grows continuously, though slowly, from the base, and wears away at the apex, but is very rarely shed entire. The only existing species in which such a process occurs regularly and periodically is FlG. 113. Head of Antelope (Gazella granti), showing Horns. From Sir V. Brooke, Proc. Zool. Soc., 1878, p. 724. the American Prong-Buck (Antilocapra), in which the horns also differ from those of all others in being bifurcated. Horns are not present at birth, but begin to grow very soon afterwards. The males of all existing Bmidse possess them, and they are also present (though usually not so fully developed) in the females of all except the genera Portax, Tragclaphus, Procapra, Antilope, ^Spyceros, Saiga, Kobus, Cervicapret, Pclca, Nanotragus, Ncotragus, and Tctraceros. 1 Another character by which the different members of the Pecora can be distinguished is derived from the characters of the molar teeth. Although there is nothing in the general mode and arrangement of the enamel folds, or in the accessory columns, absolutely distinctive between the two principal families, existing species may generally be distinguished inasmuch as the true molars of the Ccrvidss are " brachyodont," and those of the Bovidse "hypsodont," i.e., the teeth of the former have comparatively short crowns, which, as in most mammals, take their place at once with the neck (or point where the crown and root join) on a level with or a little above the alveolar border, and remain in this
i Sir Victor Brooke, froc. Zool. Soc., 1878, p. 884.