432 MAMMALIA [TILLODONTIA. position throughout the animal s life ; whereas in the other forms, the crown being lengthened and the root small, the neck does not come up to the alveolar level until a considerable part of the sur face has worn away, and the crown of the tooth thus appears for the greater part of the animal s life partially buried in the socket. In tliis form of tooth (which is almost always most developed in the pos terior molars of the permanent series), the constituent columns of the crown are necessarily nearly parallel, whereas in the first-described they diverge from the neck towards the free or grinding surface of .the tooth. In the more complete hypsodont form the interstices of the lengthened columnar folds of enamel and dentine are filled up with cementum, which gives stability to the whole organ, and which is entirely or nearly wanting in the short-crowned teeth. The same modification from low to high crowns without essential alteration of pattern is seen in an even still more marked manner in some of the Perissodactyle Ungulates, the tooth of the Horse bearing to that of Anchitherium (see HORSE, vol. xii. p. 174) the same relation as that of an Ox does to the early Selenodont Artiodaetyles. A parallel modification has been also shown to have taken place in the molar teeth of the Probosddea (see p. 423). As the hypsodont tooth is essentially a modification of and, as it were, an improvement upon the brachyodont, it is but natural to expect that all intermediate forms may be met with. Even among the Deer themselves, as pointed out by Lartet, the most ancient have very short molars, and the depressions on the grinding surface are so shallow that the bottom is always visible, while in the Cervidse, of the more recent Tertiary periods, and especially the Pleistocene and living species, these same cavities are so deep that whatever be the state of the dentition the bottom cannot be seen. Some existing Deer, as the Axis, are far more hypsodont than the majority of the family ; and, on the other hand, many of the Antelopes (as Trag- elaplms] retain much of the brachyodont character, which is, however, completely lost in the more modern and highly specialized Sheep and Oxen. Family Frontal appendages, when present, in the form of antlers. First molar at least in both jaws brachyodont. Two orifices to the lac- rymal duct, situated on or inside the rim of the orbit. An ante- orbital vacuity of such dimensions as to exclude the lacrymal bone from articulation with the nasal. Upper canines usually present in both sexes, and sometimes attaining a very great size in the male (see fig. 114). Lateral digits of both fore and hind feet almost FIG. 114. Skull of Hydropotes inermis (adult male), a Deer without Antlers, but with largely-developed upper canine teeth, x 4. From Sir V. Brooke, Proc. Zool. Soc., 1872, p. 524. always present, and frequently the distal ends of the metapodals. Placenta with _ few cotyledons. Gall-bladder absent (except in Moschus). This family contains numerous species, having a wide geographical distribution, ranging in the New World from the Arctic Circle as far south as Chili, and in the Old AVorld throughout the whole of Europe and Asia, but absent in the Ethiopian and Austra lian regions. For the characters of the generic subdivisions and their distribution, see STAG, also DEER, MUNTJAC, and MUSK- DEER. Family CAMELOPARDALID^. Frontal appendages consisting of a pair of short, erect, permanent bony processes, ossified from distinct centres, and for a time suturally connected with the frontals, though after wards ankylosed to them, covered externally with a hairy skin, present in both sexes, and even in the new-born animal. Anterior to these is a median protuberance on the frontal and contiguous parts of the nasal bones, which increases with age, and is sometimes spoken of as a third horn. No upper canines. Molars brachyodont. Lateral digits entirely absent on both fore and hind feet, even the hoofs not developed. This family contains but a single species, the well-known and very remarkable animal the Giraffe, or Camelopard (Camvlopar- dalis giraffa). See GIRAFFE. Family BOVLILE. Frontal appendages when present in the form of horns. Molars usually hypsodont. Usually only one orifice to the lacrymal canal, situated inside the rim of the orbit. Lacrymal bone almost always articulating with the nasal. Canines absent in both sexes. The lateral toes may be completely absent, but more often they are represented by the hoofs alone, supported sometimes by a very rudimentary skeleton, consisting of mere irregular nodules of bone. Distal ends of the lateral metapodals never present. Gall-bladder almost always present. Placenta with many cotyledons. The Bovidse, or hollow-horned Ruminants (Cavicornia), form a most extensive family, with members widely distributed throughout the Old World, with the exception of the Australian region ; but in America they are less numerous, and confined to the Arctic and northern temperate regions, no species being indigenous either to South or Central America. There is scarcely any natural and well- defined group in the whole class which presents greater difficulties of subdivision than this ; consequently zoologists are as yet very little agreed as to the extent and boundaries of the genera into which it should be divided. The principal species will be found more particularly described under the headings ANTELOPE, BISON, BUFFALO, CATTLE, CHAMOIS, ELAND, GNU, GOAT, HARTEBEEST, IBEX, MusK-Ox, NYLGHAU, Ox, SAIGA, and SHEEP. GROUP TILLODONTIA. Here may be noticed a remarkable group of animals, called by Marsh Tillodontia, the remains of which are found abundantly in the Lower and Middle Eocene beds of North America. They seem to combine the characters of the Ungulata, Rodentia, and Carnivora. In the genus Tillotherium of Marsh (probably identical with the pre viously described Ancliippodus of Leidy) the skull resembled FIG. 115. Skull of Tillotherium fodiens, nat. size. From Marsh. that of the Bears, but the molar teeth were of the Ungulate type, while the large incisors were very similar to those of the Rodents. The skeleton resembled that of the Carnivores, but the scaphoid and lunar bones were distinct, and there was a third trochanter on the femur. The feet were plantigrade, and each had five digits, all with long pointed claws. In the allied genus Stylinodon all the teeth were rootless. Some were as large as a Tapir. These, with other similar animals, constituting a group called Tseniodonta, are included by Cope in his large order Bunotheria, to which also the existing Insectivora are referred. The constantly increasing knowledge of these annectant forms adds to the difficulty so often referred to in this article of establishing anything like a definite classi fication of the heterodont mammals. ORDER CARNIVORA. Though the Carnivora as at present restricted x form a very natural and well-defined order among the Mammalia, it is difficult to find any important common diagnostic characters by which they can be absolutely separated ; but, as in the case of so many other natural groups, it is by the possession of a combination of various characters that 1 The FerfB of Linnaeus included all the then known species of the
modern orders Carnivora, Inseclivora, and Marsupialia.