The best illustration of the structure of the upper true molars is afforded by the figure of one of these teeth in the Proceedings of the Geological Society, May 20, 1846, published in the Quarterly Journal, vol. p. 420. " The anoplotherian character of the tooth is shown by the large size of the lobe (p, x, fig. 1), and the subgeneric peculiarity by the continua tion of its dentinal base with that of the inner and anterior lobe (id), at the early stage of attrition presented by the crown of the tooth in question. In the large and typical Anoplo theria, the lobe (p) preserves its insular form and uninterrupted contour of enamel until the crown is much more worn down than in the present tooth (fig. 1). In this respect, as in the modifications of the lower molar teeth, the genus Dichobune shows its closer affinity to the true Ruminants ; but the little fold of enamel dividing the lobe id from p distinguishes the upper molar tooth in question from that of any Ruminant" (P. 421) A new and interesting species of this genus, called Dieho bune orina, has been founded upon an almost entire lower jaw with the permanent dental series, wanting only the four middle incisors, which now forms part of the palaeontological collec tion in the British Museum. The dental formula, as shown by the mandibular teeth, and by the evidence on their crowns of the presence of the teeth of the upper jaw, is the typical one in diphyodont Mammalia, viz.—i 3-r-3 c p 44-4 m 44. The canine, with a crown like that of the first premolar, and not longer, is separated from it by an interval of half the breadth of the crown, and by a narrower interval from the outer incisor. The first premolar is divided by an interval of scarce a line's breadth from the second. The rest of the molar series are in contact. The total length of the lower jaw is 5 inches 11 lines (0•148) ; that of the molar series is 2 inches 11 lines (0'075) ; that of the three true molars is 1 inch 4i lines (0•035). The near equality in height of the crowns of all the teeth, and their general character, show that the animal belonged to the anoplotherioid family. The dentition of the present species differs from that of Diciwdon in the absence of the accessory cusps on the inner side of the base of the true molars ; and both from Diciwdon cuspidatu,s and Xiphodon gracilis in the minor antero-posterior extent of the premolars ; it corresponds with Dichobune (as represented by the D. leporina, Cuvier) in the proportions of the premolars and in the separation of the canine from the adjoining teeth : to this genus, therefore, the fossil is referable, provisionally, in the absence of knowledge of the molars of the upper jaw, which are the most characteristic : and the writer has proposed to call the species, from the size of the animal repre sented by the fossil, Dichobune mina. It is from Hampshire eocene.
XnmonoN.—The genus Xiphodon was indicated, and its name proposed, by Cuvier, for a small and delicate, long and slender-limbed, anoplotherian animal, which, in his first Memoir (Annales du Museum, tom. iii., p. 55, 1803), he had called Anoplotheriutn medium ; but he altered the name, in the second 4to edition of the Ossemens Fossiles (tom. iii., pp.
69 and 251, 1822), to that of Anoplotherium gracile.
The distinction indicated by Cuvier is now accepted by palaeontologists as a generic one, and a second species (Xipho don Geylensis) has been added by M. Gervais (Paleontographie Francaise, 4to, 1845, p. 90) to the type-species, Xiphodon gracilis, of which he figures an instructive portion of the den tal series of both jaws, obtained from the lignites of Debruge near Apt. The dental formula of Xiphodon is the typical one, , 44.
The teeth are arranged in a continuous series in both jaws. The canines and first three premolars have the crowns more extended antero-posteriorly, lower, thinner, transversely, and more trenchant, than in the type Anoplotheria (whence the name Xiphodon, or sword-tooth). The feet are didactyle, with metacarpals and metatarsals distinct. The tail is short. The lower true molars have two pairs of crescentic lobes with the convexity turned outwards. It was nearly allied to Dichodon. MICROTHERIUM.—Entire crania of Microtherium, from the lacustrine calcareous marls of the Puy-de-Dame, are in the British Museum, and these show that the hinder division of the upper true molars was complicated by the additional (third) cusp.
With regard to Microtherium, the unusually perfect fossil skulls of that small Herbivore, which did not exceed in size the delicate chevrotains of Java and other ludo-Archipelagic islands—e. g. Tragulus kanelta—are of importance in regard to the question of their alleged affinity to the Ruminantia, on account of the demonstration they give of the persistent and functional upper incisor teeth. The little eocene even-toed Herbivores, like the larger Anoplotherioids, thus departed from the characters of the true Ruminants of the present day, in the same degree in which they adhered to the more general type of the artiodactyles. Had M. de Blainville, who believed them to be Ruminants, possessed no other evidence of the Mieroth.erium than of the Dichobune murina and Dichobune obliqua, Cuv., he would have had the same grounds for refer ring the Microth,eria„ as the Dichobunes, to the genus Tragulus or Moechus (les Chevrotains); but the entire dentition of the upper jaw of the species Anoplotherium murinum and A. obliquum, referred by Cuvier to his genus Dichobune, must be known before the existence of Ruminants in the upper eocene gypsum of Paris can be inferred.
No doubt the affinity of these small Anoplotherioids to the Chevrotains was very close. Let the formative force be trans ferred from the small upper incisors to the contiguous canines, and the transition would be effected. We know that the ruminant stomach of the species of Tragulus is simplified by the suppression of the psalterium or third bag. The stomach of the small Anoplotherioids, whilst preserving a certain degree of complexity, might have been somewhat more simplified. The certain information which the gradations of dentition dis played by the above-cited extinct species impart, testifies to the artificial character of the order Ruminantia of the modern systems, and to the natural character of that wider group of even-toed hoofed animals for which has been proposed the term