ORDER UNGULATA. 



315 



more simple in structure than the true molars ; while the last lower 

 true molar nearly always has a third lobe, the same feature being 

 repeated in the last lower tooth of the milk series. 



In the Pecora the pelvis (fig. 1 1 2 8 bis) is characterised by the 

 great length of the ilia, which are not much expanded ; but in the 

 Hippopotamus these bones are shorter and more expanded, and thus 

 approximate to those of the Perissodactyla. The symphysis of the 

 ischia and pubes is much elongated. 

 The true molars in the upper jaw 

 carry either four or five main columns ; 

 in the less specialised forms such as 

 the Pigs (figs. 1 1 94, 1 195), such 

 columns in both upper and lower 

 molars form low subconical tubercles, 

 when the dentition is termed buno- 

 dont ; but in others, such as Eporeo- 

 don (fig. 1 201) and the Ruminants 

 (fig. 1220), the outer pair of columns 

 becomes transversely flattened, and 

 the inner pair crescent-shaped, and 

 the dentition is then termed seleno- 

 dont. In the lower molars of the 

 latter type of dentition it is the inner 

 columns that become flattened, while 

 the outer ones assume a crescent 



shape. There is, however, a complete passage from the one type 

 to the other ; the most specialised forms with a hypso-selenodont 

 dentition being of comparatively recent origin. The existing mem- 

 bers of this suborder are divided into the sections Suina (Pigs 

 and Hippopotamus), Tylopoda (Camels), Tragulina (Chevrotains), 

 and Pecora (typical Ruminants) ; but since such divisions will not 

 hold good for the fossil forms, it will be convenient merely to 

 divide the suborder into families. 



It should, moreover, be observed that it is by no means certain 

 that all the above-mentioned characters will apply to some of the 

 less known earlier members of the suborder, since some of these 

 included in the Anoplotheriidce present certain remote indications of 

 affinity with the Perissodactyla. 



The advance from a bunodont to a hypso-selenodont dentition is 

 accompanied in this suborder by a tendency in the second and third 

 metapodials to coalesce into a cannon-bone, and also by a change in 

 the form of the odontoid process of the axis vertebra from a peg-like 

 projection to a spout-like demicylinder. The earliest form which 

 has been referred to this suborder is the small Pantolestes, from the 

 Lower Eocene of the United States. This genus is still very imper- 



Fig. 1186. — Left astragalus of a Rumi- 

 nant Artiodactyle (Bos). Reduced. 



