130 COPE 



three behind, to one on all the feet (the horse). They may 

 be considered under two heads with respect to the structure 

 of their superior molars, viz.: first those in -which the exter- 

 nal wall of the crown does not form two Vs (Rhinocero- 

 toidea), and those in which such Vs are present, with the 

 angles directed inwards (Equoidea). Each series includes 

 several families, mostly extinct ; in the former the rhinoceros 

 (Fig. 63) and tapir, and in the latter the horse, are still 

 living. The earliest forms belong to the former division, 

 and some of them (Hyracotherium, Fig. 64) have the super- 

 ior molars almost quadritubercular. They appear first in 

 the lowest Eocene. Three-toed horses first appear in the 

 Lower Neocene, and one-toed horses in the Upper Neocene. 

 The Artiodactyla are represented by a great variety of 

 forms, which differ primarily in their dental characters. 

 Thus the oldest type (Trigonolestoidea) has tritubercular 

 superior molars. Of the remainder, one series (Suoidea) 

 have the molars quadritubercular, or more highly tubercu- 

 lar. The remaining types have the tubercles of the molars 

 more or less flattened on one side, so as to give crescentic 

 figures on section (Fig. 65), and are hence called seleno- 

 dont. In some of these there are five crescents (Anthraco- 

 theroidea) ; in the other there are only four. The latter may 

 have most of the premolars simple (Cameloidea, Fig. 65) or 

 complex (Booidea). Some types of the former and all of the 

 latter lack superior incisor teeth, and have the cuboid and 

 navicular bones coossified. The Booidea culminate in forms 

 with horns or bony processes of. the skull, which may be 

 permanent (Bovidse, or annually shed (Cervidse). These 

 divisions appear in geological time in the order of structu- 

 ral modification as here mentioned. This appearance is 

 represented, together with the divisions of the Perissodac- 

 tyla, in the following table : 



