THE RODENTS 191 



rence of anterior and posterior cingula or accessory ridges 

 running' laterally, the anterior terminating in a small buccal 

 parastyle, the posterior fading away as it nears the metacone. 

 There is no hypocone. In the case of the third molar the meta- 

 cone region is enlarged so that this tooth is the largest of all, 

 a character which may he primitive since the molars increase 

 progressively in size from before backward but on the other 

 hand, may be secondary indicating the commencement of the 

 well recognized herbivorous specialization previously noted 

 on several occasions. The crowns of the lower molars are 

 rhomboidal with a basin-shaped talonid presenting hypoconid 

 and entoconid. The anterior edge of the tooth shows a small 

 lateral protoconid and a larger lingual cusp, the homology of 

 which is obscure for its position indicates that it may repre- 

 sent the paraconid, the metaconid or these two cusps fused to- 

 gether. Here, again, the last tooth is the largest. 



The rhomboidal character of the lower molars is explained 

 by the obliquity of the masticatory movements. Antero-pos- 

 terior motion of the jaws can only take place when the incisors 

 alone are used in gnawing. In such movements the incisors 

 by mutual grinding come to bevel each other upon the pos- 

 terior face ; the molars are not in occlusion. C4reat elongation 

 of the glenoid fossa in the antero-posterior direction permits 

 free gnawing movement and the lips, sinking deeply their in- 

 ternal hair covered surfaces into the hiatus between incisors 

 and cheek teeth, prevent any chips entering the mouth proper. 



In spite of the fact that the AVoodchuck is largely a grass 

 feeder its molar teeth are brachyodont and do not possess per- 

 manent tooth pulps. These features it shares with its Old- 

 World relations, the food of which consists of roots, leaves 

 and the seeds of various plants. 



In the photograph of the dentition of the common Brown 

 Eat, Mus norvegicus (Fig. 68) representing the Myomorpha, it 

 is seen that the tooth formula is: 



