DICHODON. 369 



break — a character which is only manifested by mankind 

 among existing Mammals — the crowns of the teeth, in Dicho- 

 don, being all of nearly equal height, as they are in man. 

 On each side of both upper and lower jaws there are in the 

 Dichodon (fig. 1 30) three incisors (i, i, z, 3), one canine (c), four 

 premolars (p, i, z, 3, 4), and three true molars (m 1, 2, 3) — in all 

 forty-four teeth, constituting the typical diphyodont dentition 

 which so many mammalian genera, on their first appearance 

 in the eocene strata, exhibit. It is formulized as follows : — 

 * §> c T5>P S» m 13 = 44. From the first incisor to the third 

 premolar the teeth have a more or less trenchant crown. The 

 back of the third premolar (p 3) and all the fourth premolar 

 (p 4), shew the crushing form of crown ; the pattern of which 

 in the true molars, after the wearing down of the first sharp 

 cusps, produces the double crescentic lines of enamel which 

 are now peculiar to the Euminants amongst hoofed quadrupeds. 

 The first (p l )> second (p 2), and third (p 3) premolars have 

 their crown much extended from before backwards, with three 

 progressively more developed and pointed compressed cusps 

 on the same line : to which is added, in the upper jaw, an 

 inner ridge, developed in the third premolar (j> 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 (m 1, 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 (m i, 2, 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 direction to those above. 

 At the upper part of fig. 130 the outer side of the true molars, 

 of the last premolar, of the canine, and of the incisors, is shewn, 

 together with the grinding surface of the three anterior pre- 

 molars in the upper jaw. Below these the inner surface of 



2 b 



