UNGULATA. 339 



artiodactyls have now been discovered, that it is impossible 

 any longer to define these four groups when treating of the sub- 

 order as a whole. The arrangement to be provisionally adopted 

 must correspond in a more or less vague way with the lines 

 of descent which it seems possible to recognize as the successive 

 extinct genera are compared one with another. All the 

 members of the sub-order agree in being digitigrade quad- 

 rupeds, with the axis of both feet passing between digits 

 nos. in and iv, which are almost equally developed (hence 

 parcuKonic) ; and the bones of the two series in the carpus 

 alternate or interlock, while the astragalus in the tarsus is 

 always convex below, articulating about half with the navicular, 

 half with the cuboid (fig. 181 B, p. 319). The number of the 

 dorso-lumbar vertebrae, so far as known, is curiously constant, 

 namely, nineteen. There is no clavicle. The femur is destitute 

 of a third trochanter ; and the fibula always articulates with 

 the calcaneum, even when it is reduced to a mere rudiment. 

 The premolar teeth always remain simpler than the molars. 

 In other features of the skeleton many changes are to be 

 observed as the artiodactyls are traced upwards from the 

 Lower Eocene to the present day, both in Europe and North 

 America. So far as known, the earliest forms possess four 

 or five complete digits on each foot (fig. 195 A) ; and even 

 when these digits are reduced to one pair, they at first remain 

 separate throughout the life of the animal (fig. 195 B). The two 

 metapodials are fused together into a cannon bone (fig. 195 c, D) 

 for the first time in the Upper Eocene (Prodremotherium), but 

 such a consolidation of the foot is not common until the 

 Lower Pliocene. When this stage is reached the ulna is much 

 reduced and often fuses with the radius ; while the fibula is 

 eventually represented solely by its lower end, which continues 

 to articulate with the calcaneum. Again, in the earliest artio- 

 dactyls the odontoid process of the axis vertebra is a simple 

 peg, while in the later more specialized forms it becomes 

 spatulate or semi-cylindrical in shape. In the skull, the facial 

 region often bends downwards upon the cranial region, and the 

 latter becomes thickened to bear horns or antlers. The 

 grinding teeth are at first low-crowned (brachyodont, fig. 195 E), 



222 



