362 THE VERTEBRATE SKELETON. 



characters are little specialised, and they show relationship on 

 the one hand to the Ungulata vera and on the other to the 

 Hyracoidea. They also have characters allying them to the 

 Carnivora. They generally have the typical mammalian 

 series of forty-four teeth, the molars being brachydont and 

 generally bunodont. The premolars are more simple than the 

 molars. The limbs are plantigrade, and have five digits with 

 rather pointed ungual phalanges. The os magnum, as in living 

 Subungulates, articulates with the lunar, not reaching the 

 scaphoid. The astragalus has an elongated neck, a pulley-like 

 proximal and a convex distal articular surface, and does not 

 articulate with the cuboid. The humerus has an ent-epicondylar 

 foramen, and the femur has a third trochanter. The best 

 known genus is Phenacodus ; it is perhaps the most primitive 

 ungulate whose skeleton is thoroughly well known, and is of 

 special interest from the fact that it is regarded as the lowest 

 stage in the evolutionary series of the horse. Its remains 

 are found in the Lower Eocene of Wyoming. 



Suborder (3). HYRACOIDEA 1 . 



This group of animals is very isolated, having no very close 

 allies, either living or extinct. The digits are provided with 

 flat nails, except the second digit of the pes, which is clawed. 

 Canine teeth are absent, and the dental formula is usually 



given as i -, c ^, pm -, ra ^. The upper incisors are long 



and curved, and have persistent pulps as in Rodents ; their 

 terminations are, however, pointed, not chisel-shaped, as in 

 Rodents. The lower incisors have pectinated edges. The 

 grinding teeth have a pattern much like that in Rhinoceros. 

 In the skull (fig. 83) the postorbital processes of the frontal 

 and jugal almost or quite meet. The jugal forms part of the 



1 See 0. Thomas, "On the species of Hyracoidea," P. Z. S., 1892, 

 p. 50. 



