RHINOCEROS, L.—The rhinoceros, like the elephant, was represented in pliocene and pleistocene times, in tempe rate and northern latitudes of Asia and Europe, by extinct species. One (Rhinoceros leptorhinus) associated with the Hippopotamus major in fresh-water pliocene deposits ; another (R. tichorrhinus) with the mammoth in pleistocene beds and drift. The discovery of the carcase of the tichorrine rhino ceros in frozen soil, recorded by Pallas in his " Voyages dans l'Asie showed the same adaptation of this, at present tropical, form of quadruped to a cold climate, by a twofold covering of wool and hair, as was subsequently de monstrated to be the case with the mam moth. Both the above-named fossil rhinoceroses were two-horned ; but they were preceded, in the pliocene and miocene periods, by species devoid of horns, yet a rhino ceros in all other essentials (Acero therium, gaup).
The tions which the upper molars of the rhinoceros present as compared with those of its antetype, the Palotherium, will be readily understood by comparing fig. 99 with fig. 122, and are as follows :—The concavities (f f) on the outer side of the crown, in fig. 99, are almost levelled, and from one of them a slight convexity projects, in some species of Rhinoceros, giving a gently undulated surface to that side of the tooth. The valley (e) is more expanded at its termination (i), in the Rhinoceros ; and, in some species, it bifurcates and deepens, so that one branch may form an insulated circle of enamel when the crown is worn. The posterior valley (g) is usually deeper and more extended. The ordinary lobes (a, b, c, d) are very similar, and produce, by the confluence of a with c, and of b with d, the two oblique tracts of dentine which are more decidedly esta blished as transverse ridges in the Lophiodont or Tapiroid group. A basal ridge (r) girts more or less completely the inner and the fore and hind parts of the base of the crown.
Not fewer than twenty species of extinct rhinoceroses are entered in Palaeontological catalogues.
The extinct Chceropotainus, Anaracotherium, Hyopotamus, and Hippohyus, had the typical dental formula, and this is preserved in the existing representative of the same section of non-ruminant Artiodactyles, the hog. The first true molar when the permanent dentition is completed, exhibits the effects of its early development in a more marked degree than in most other Mammalia, and in the Wild Boar has its tubercles worn down and a smooth field of dentine exposed by the time the last molar has come into place ; it originally bears four primary cones, with smaller sub-divisions formed by the wrinkled enamel, and an interior and posterior ridge. The four cones produced by the crucial impression, of which the transverse part is the deepest, are repeated on the second true molar with more complex shallow divisions, and a larger tuber culate posterior ridge. The greater extent of the last molar is chiefly produced by the development of the back ridge into a cluster of tubercles ; the four primary cones being distin guishable on the anterior main body of the tooth. The crowns of the lower molars are very similar to those above, but are rather narrower, and the outer and inner basal tubercles are much smaller, or are wanting ; the grinding surface of the last is shown in fig. 123.
Extinct species of hog have been found in miocene beds at Eppelsheim (Sus palceochcerus, Kp.), and at Simorre (S. simor rensis,Lt.) ; in pliocene beds (S. arvernensis, Crt.), and in pleis tocene and later deposits, where the species (S. sorofa fossilis) is indistinguishable from the present wild boar.