DICHODON, Ow.—The upper eocene beds of Hamp shire have yielded evidence of an extinct form of even-toed (artiodactyle) hoofed quadruped, most interesting as a transi tional form between the Anoplotherioids and the true Rumi nants. Like the Anoplotherium the dental series is continu ous, without break—a character which is only manifested by mankind among existing Mammals—the crowns of the teeth, in Dielwdon, being all of nearly equal height, as they are in man. On each side of both upper and lower jaws there are in the Diekdon (fig. 102) three incisors (i 1, 2, 3), one canine (c), four premolars (p 142210, and three true molars (m1, 2, 3) —in all forty-four teeth, constituting the typical diphyo dont* dentition which so many mammalian genera, on their first appearance in the eocene strata, exhibit. It is formalized 3_-3 I-1 8-3 88 follows :—i C1.1,p Et 73 --=-- 44. From the first incisor to the third premolar the teeth have a more or less trench ant crown. The back of the third premolar (p 3), and all the fourth premolar (p. 4), show the crushing form of crown ; the pat tern of which in the true molars, after the wearing down of the first sharp cusps, pro duces the double crescentic lines of enamel which are now peculiar to the Ruminants amongst hoofed quadrupeds. The first (p 1), second z), and third (p 3) premolars, have their crown much extended from before back wards, with three progressively more developed and pointed compress ed cusps on the same line : to which is added, in the upper jaw, an inner ridge, de veloped in the third premolar (p 3) into an inner posterior cusp. The fourth premolar (p 4) has a thicker and shorter crown with two pairs of cusps. The upper true molars (mi, 2, 3) have the two pairs of cusps sharp and pointed, with a series of five low accessory points developed from the outer part of the cingulum. The lower molars (n', z, 3) have as complex crowns as the upper ones, but with the accessory basal points (a, b, c, e) developed from the inner, instead of the outer side of the crown, and with the convex sides of the chief cusps turned in the opposite direc tion to those above. At the upper part of
fig. 102 the outer side of the true molars, of the last premolar, of the canine, and of the incisors, is shown, together with the grinding surface of the three anterior premolars in the upper jaw. Below these the inner surface of the entire series of the lower teeth is shown, together with the grinding surface of the three true molars, the last of which (m 3) here supports a third pair of lobes (e.) As compared with the anoplotherian molar (fig. 100), the outer lobes (a, b) of that of the Dichodon (fig. 103) are thicker and sharper ; the inner ones (e, d) —especially the latter—are developed to an equality with the outer ones, and more distinctly separated from them. The valley (m) extends across the whole breadth of the tooth, and is crossed at right angles by the fore-and-aft doubly-curved valley (g and The extinct species showing the above characters, and on which the genus was founded,* was nearly the size of a fallow deer : it is called Dichocion cuspidalus, in reference to the num ber of sharp points on the unworn molars. The dentition indicates that its food may have been of a peculiar character, perhaps not exclusively of a vegetable nature.
In the same upper eocene formation of Hampshire have been found instructive examples of some smaller members of the extinct anoplotherioid family.