THE SKELETON IN MAMMALIA. THE TEETH. 429 



first premolar is separated by a long diastema from the 

 others. 



The Tragulina or Chevrotains have no upper incisors, while 

 the canines are largely developed, especially in the male. 



The Ruminantia or Pecora are very uniform as regards 

 their dentition. The upper incisors are always absent, for 

 though their germs are developed they are reabsorbed without 

 ever becoming visible, and as a rule the upper canines are 

 absent too, while the lower canines are incisiform. The 

 grinding teeth are typically selenodont, and in the lower jaw 

 form a continuous series separated by a wide diastema from 

 the canines. The dental formula is usually 



.0 01 3 3 



The canines are largely developed in the male Muskdeer 

 (Moschus) and in Hydropotes. 



PERISSODACTYLA. The premolars and molars have a very 

 similar structure and form a continuous series of large 

 square teeth with complex crowns. The crowns are always 

 constructed on some modification of the bilophodont 1 plan, as 

 is easily seen in the case of the forms with brachydont teeth, 

 but in animals like the Horse, in which the teeth are very 

 hypsodont, this arrangement is hard to trace. All four pre- 

 molars in the upper jaw are preceded by milk teeth, while in 

 Artiodactyla the first has no milk predecessors. 



In the Tapiridae the grinding teeth are brachydont and 

 the lower ones are typically bilophodont. The last two upper 

 molars have the transverse ridges united by an outer longi- 



31 43 



tudinal ridge. The dentition is i ^ c =- pm -m-^, total 42. 



o 1 o o 



In some of the extinct Perissodactyles such as Lophiodon 2 , 



1 See p. 345. 



2 According to H. F. Osborn, Amer. Natural., xxvi. p. 763, a number of 

 not very closely allied forms have been included under Lophiodon. 



