A large extinct carnivorous animal (Machairodus, fig. 293, VI.), had the upper canine teeth, c, developed to almost the same disproportionate length as in the Walrus, whereby they were also compelled to pass outside the lower jaw when the mouth was shut. But these teeth were shaped after the type of the feline canines, only with more compressed and trenchant crowns; and they were associated with other teeth in number and kind demonstrating the feline affinity of the genus Machairodus. Its remains occur in newer tertiary deposits and in caves.1

In older tertiary formations, remains of carnivorous Mammals have been found with the three true molar teeth as expressly modified for the division of flesh, and as worthy the term of 'sectorials' as the teeth so called in the lion. These teeth were associated with conical premolars, long canines, and short incisors, so as to exemplify the typical formula, e.g.—

4.4; 3.3

The extinct Hyænodon and Pterodon of the upper eocene formations of Hampshire and of France, manifest this interesting and instructive character of dentition.

266

A reduced view of the lower jaw of the Hyanodon Requieni is given in fig. 266. After the canines, c, come four successively enlarging conical compressed premolars, p 1-4; then, instead of a single carnassial representing the first true molar, there are three of these singularly modified teeththe first, m 1, being of suddenly small size, as compared with the antecedent premolar, and obviously illustrating its true nature as a continuation of the deciduous series, with which, doubtless, it agreed in size. It became a permanent tooth only because there was no premolar developed beneath it, so as to displace it. The succeeding carnassial true molars, m 2 and 3, progressively increase in size. The symbols in fig. 266 denote the homologies of the teeth. The marks of abrasion on the lower teeth in the Hyanodon prove the upper series to have been the same in number.

A second form of equally ancient Carnivore was a mixed1 Kent's Hole, Devonshire, e. g.; cxvi". p. 174.

feeding animal, allied to the viverrine and canine families, the true molars presenting the tuberculate modification, and the typical number and kinds of teeth being functionally developed, as in the Hyanodon. The series in the upper jaw are shown in The term tubercular' is as applicable to the three true molars of the Amphicyon, m 1, 2, 3, as the term 'carnassial' is to those of the Hyanodon.

fig. 267.

§ 221. Teeth of Ungulata.-The most common characteristic of this dentition is the large size, cuboid shape, and complex structure of the crowns of the grinding teeth. The enamel not only incloses but dips or penetrates into the substance of the dentinal body, and the cement, which is thick, accompanies the enamel. Thus the massive grinding organ is made up of substances of different densities, and the working surface is irregular by the projections of the harder material, as in the mineral grit' that is thereby suitable as a millstone.

A. Homologies of the parts of the grinding surface. The pattern of the grinding surface, especially of the upper molars, varies in each genus of Ungulata, and is eminently characteristic thereof. Nevertheless, two leading types may be recognised. One, of unsymmetrical character, was early shown in Palæotherium, and is traceable in secondary modifications characteristic of Paloplotherium, Hipparion, Equus, Hyrax, and Rhinoceros. A second was as early manifested in Anoplotherium and Dichodon; it is more symmetrical in pattern, and is traceable, with modifications, in Dicotyles, Sus, Hippopotamus, and Ruminants. Indications of a more generalised type of molar have been obtained from tertiary

2

In

deposits antecedent in time to those characterised by Paleoor Anoplo-therium: they are afforded afforded by Pliolophus,' and Coryphodon. The answerable parts of the grinding surface will first be illustrated in the unsymmetrical series. Palæotherium, e. g. fig. 268, the tract of dentine, a, b, extending along the outer side of the crown, has two indents, f, f, whereby it is divided into two lobes, an anterior or ant-external lobe,' a, and a posterior or post-external lobe,' b. The tract of dentine along the inner side of the crown is also divided by two deeper and more oblique clefts or valleys into an ant-internal lobe,' c, m, and a post-internal lobe,' d: these lobes extend obliquely inward and backward from the outer ones, of which they are direct con

6

tinuations. The anterior of the two inner clefts, e, i, extends from the middle of the inner surface of the crown obliquely outward and forward: the posterior one, g, h, enters at the posterior side of the crown, and extends nearly parallel with e, i: both valleys expand and deepen at their blind ends. At an early period of the attrition of the crown they intercommunicate, and extend to the anterior side of the crown, at 1, as in the younger molar of Paloplotherium, fig. 269. But the shallow communicating passages between h and i, i and l, are soon obliterated, the dentine of lobe d becoming continuous with b; and that marked e with a. In Paloplotherium a branch valley, also, extends from e i, to the anterior side of the crown, k, cutting off the part of the ant-internal lobe m from the rest of c; but, by continued abrasion, this valley is also obliterated, and the tooth assumes more of the palæotherian pattern. In Equus, fig. 270, the valleys are of less equal depth than in Palæotherium, and are 1 cxv". p. 54. 2 CXVI". p. 299.

so shallow midway that, at an early stage of attrition, the entry of the posterior valley, g, is separated from its termination, h; and that of the internal valley, e, from its termination i; the blind ends of both valleys, moreover, are more extended and irregular, than in Palæotherium, with the tendency to curve, so as to produce the crescentic form of the islands, i, h, in fig. 270. The obliteration of the mid-part of the accessory valley, k, unites the dentinal tract, m, to the rest of the lobe, c, as in Palæotherium, fig. 268: but it long remains separate in Hipparion, as in Paloplotherium, fig. 269.

The Rhinoceros and Hyrax more closely adhere to the Palæotherium type: but the outer indents, f, f, are less marked. The

horse approaches nearest to the symmetrical type of the Ruminants, in which the homologous parts of the crown can, mostly, be well defined.

In the unworn crown of the Ruminant molar, fig. 271, the valley, g, h, extends across the crown more parallel with the long axis of the jaw, than in fig. 268, curving with the concavity outward: it communicates with the valley, i; and, as in Paloplotherium, this is continued to the foreside of the crown, as at 1, fig. 269, severing the lobe e from a. In Ruminants, both the anterior and posterior entries to this antero-posterior doublecurved cleft are so shallow that they are soon obliterated, and the lobe b is continued by a tract of dentine, with d, along the hind part of the crown: as the lobe a is continued into lobe c at the fore part, as seen in the worn molar of the deer, fig. 271: the middle of valley, e, is separated from the end i, as in the horse: but the course of this valley is more transverse, and more directly bisects the antero-posterior valley, h, i: thus the inner lobes c and d are more parallel with and similar to the outer

lobes a, b. Whether the accessory lobule m, be a homologue of the end, so marked, of lobe e in Palæotherium, Paloplotherium, and Equus, or a special development at the entry of valley e may be doubtful.

272

In the Hippopotamus, fig. 272, the valley commencing at the inner side of the crown at e, extends straight across the tooth to n, bisecting the crown transversely it is also bisected, antero-posteriorly, by a shallower valley, answering to h, i, fig. 271. At the stage of attrition shown in fig. 272, the remnant of the latter valley is seen at h and i: the deeper transverse valley, e, n, remains: the shorter indents, f,f,g, k, give the trefoil character to the

two chief divisions of the crown characteristic of Hippopotamus. Another exposition of the homologous parts of the complex crowns of the Ungulate molars assumes the crucial division into four quarters or lobes to be the primitive modification. The foreand-aft cleft has already begun to be filled by the mid-lobules in Pliolophus: the arrest of the outer end of the transverse cleft produces the continuity of a with b: that of its mid-part, of d with e: the obliteration of both ends of the antero-posterior cleft insulates that cleft, as in the Ruminant. The obliteration of the middle of the transverse cleft produces the continuation of a, b, with d, c; while the oblique continuation of e with i, and the retention of the continuity of g with h, leads to the type of Palæotherium and Rhinoceros.

A sub-type of grinding surface is produced by the existence of a transverse without an antero-posterior valley, dividing the crown into a pair of transverse ridges; as in the Tapir; which, however, is mainly the greater development, and more transverse disposition, of the tracts b, d, and a, c, in Palæotherium, fig. 268. The 'bilophodont' sub-type becomes more marked in Dinotherium, fig. 288, and in the anterior small molar of Mastodon: the successive multiplication of the transverse ridges completes the transition into the molar character of Elephas.

B. Artiodactyla. The extinct Charopotamus, Anthracotherium, Hyopotamus and Hippohyus, had the typical dental formula, and this is preserved in the existing representative of the same section