ORDER UNGULATA. 1313 



lunar bones of the carpus are always distinct ; but the radius and 

 ulna may unite. 



In the great majority of instances the cheek-teeth are rooted, and 

 in the suborders Artiodactyla, Perissodactyla, and Proboscidea, which 

 comprise the most specialised members of the order, a gradual in- 

 crease in the height of the crowns of these teeth may be traced 

 from the generalised to the specialised genera. Those teeth in 

 which the crowns are low, and their whole structure is visible from 

 the grinding surface, being known as brachydont (compare fig. 1 195); 

 while those with high crowns, in which the bases of the enamel-folds 

 are invisible from the grinding surface, are termed hypsodont (fig. 

 1220). The change from a brachydont to a hypsodont dentition 

 is accompanied by the production of a nearly flat and horizontal 

 grinding surface in the cheek-teeth, in place of a more or less 

 strongly ridged one ; the more specialised type being adapted for a 

 perfect grinding action of the upper against the lower teeth, while 

 in the more generalised type the action is to a great extent a snap- 

 ping one. Examples of the former type are shown by the Horse 

 and Ox, and of the latter by the Pig and Hyrax. Hypsodontism 

 is not confined to this order, as will be noticed in the sequel. 



Some remarks on the probable origin of the order are made 

 below under the head of the Condylarthra. In the specialised 

 forms there is very often a tendency to a suppression of the anterior 

 teeth, more especially in the lower jaw. 



Suborder i. Artiodactyla. — This and the next suborder 

 present certain structural modifications of the extremities, by which 

 they are distinguished from the remaining five suborders, and on 

 which account, as already mentioned, they are grouped together by 

 some writers under the name of Ungulata Vera, or Diplarthra. 

 Thus the feet are never plantigrade, and the number of functional 

 digits does not exceed four. In the carpus (fig. 1185) the scaphoid 

 is supported by and largely articulates with the magnum ; while the 

 latter (together with the unciform) supports the lunar, and has no 

 connection with the cuneiform. In the tarsus the cuboid extends 

 inwardly to articulate with the astragalus, which is deeply grooved 

 (fig. 1 186). All the component bones of both the carpus and 

 tarsus strongly interlock, which makes the structure of these joints 

 more complex than in the other suborders. Finally, the jugal forms 

 the anterior part of the zygomatic arch (fig. 1187), and the brain is 

 of relatively large size and complex structure. 



This suborder, so far as at present known, is distinguished 

 from the Perissodactyla by the distal surface of the astragalus (fig. 

 1 1 86) being ginglymoid, by the third and fourth digits (fig. 1185) 

 being equal in size, and arranged symmetrically on either side of 

 a line drawn between them ; by the absence of a third trochanter 



