6 9 8 



PROTO UNGULATES 



XXVIII. 2- 



square, by loss of the paraconid and raising the heel, whose outer 

 hypoconid and inner entoconid make a pair, behind the metaconid 

 and protoconid (Fig. 467). Even more characteristic is the change in 

 the cusps themselves. Instead of the original sharp points they develop 

 first low cones, giving so-called bunodont grinding surfaces. Then, in 

 later evolutionary stages, ridges or lophs appear between the cusps; 

 an ectoloph between paracone and metacone, transverse protolph at 

 the front of the tooth (between protocone and protoconule), and 

 metaloph behind between hypocone and metaconule. All sorts of 



B 



Fig. 467. Hyracotheriwn. Upper (a) and lower (b) premolars and molars. 



(After Wortmann from Outlines of Vertebrate Palaeontology, Cambridge 



University Press.) 



further developments and cross-connexions may then take place in 

 such lophodont molars; moreover, the whole tooth becomes sur- 

 rounded by 'cement' (bone), so that the ridges are supported as they 

 wear away and continually maintain a rough surface for grinding. 

 Short (brachydont) molars, which would wear away too quickly, are 

 replaced by deep (hyposodont) ones, which grow continually from 

 open roots in extreme instances. 



In these animals that need to increase the grinding surfaces the 

 whole set of teeth is usually retained and the molar structure extends 

 forwards to the premolars. This molarization may be said to be 

 the opposite of the condition in carnivores, where the tooth row is 

 shortened and the hinder teeth come to have cutting edges like the 

 front ones. The incisors of ungulates become specialized for cropping 

 the food; in artiodactyls the upper ones are lost and the lower work 

 against a horny upper lip. The canine is often absent, leaving a 

 diastema. The cropping and grinding mechanisms involve various 

 modifications of the lips, palate, tongue, and, of course, the jaws and 

 their muscles. The articulation of the jaw with the skull is usually made 



