3G0 PALEONTOLOGY. 



the horse, hyrax, hog tribe, and anoplothere, and differs from 

 the rhinoceros, tapir, and pakeothere, which have the maxil- 

 laries, as well as the nasals and premaxillaries, entering into 

 the formation of the external bony nostril. 



The ungulate and herbivorous character of Pliolophus is 

 most distinctly marked by the modifications of the lower jaw, 

 especially by the relative dimensions of the parts of the 

 ascending ramus which give the extent of attachment of the 

 biting (temporal) and grinding (masseteric and pterygoid) 

 muscles respectively. In the shape of the mandible Plio- 

 loplius most resembles Tapirus among existing, and the 

 Pcdceotherium among extinct, Mammals. As in almost every 

 species of eocene quadruped yet discovered, the Pliolophus 

 presents the type-dentition of the placental diphyodont 

 series, viz. — 



.3-3 1-1 4-4 3-3 _ u 



* 3 _ 3 > Cf=i> ^4=4' ^3-3-**- 



These symbols signify that there are 3 incisors, 1 canine, 

 4 premolars, and 3 molars, on each side of both upper and 

 lower jaws, making 44 teeth in all. The " incisors" are the 

 teeth implanted in the premaxillary bones (fig. 121, 22), and in 

 the opposed end of the mandible ; the " canine" is the tooth 

 in the maxillary (ib. 21) nearest the suture with 22, and the 

 opposing tooth in mandible : it is usually long and pointed. 

 The "premolars" are those teeth, at the fore part of the grind- 

 ing series, which succeed vertically (fig. 122, p 1,2,3,4), and 

 displace the deciduous or milk-molars (ib., d i, 2, 3, 4). The 

 "molars" are those at the back part of the grinding series 

 which succeed the milk-molars and each other horizontally, 

 one behind another, without displacing any predecessor (ib., 

 m 1, 2, 3). In all non-marsupial mammals — the majority of the 

 class — which have two sets of teeth, milk and permanent, the 

 several kinds do not exceed the numbers above defined ; but 

 only one or two genera, e.g., Sus and Gymnura, now exhibit 



