666 CASTORID^ 



By degrees the deep portion of the masseter lateralis muscle 

 became the principal means of moving the lower jaw during 

 gnawing, and it gradually extended its area of origin upwards 

 far above the level of the infraorbital foramen, and forwards to 

 a greater or less extent over the surface of the ascending 

 branch of the premaxilla. This muscle thus usurped much of 

 the function of the masseter medialis ; the latter therefore 

 declined in importance and was gradually driven away from the 

 infraorbital canal ; the canal consequently was straitened, and 

 resumed its normal mammalian status as a mere conduit for 

 the facial nerve and vessels. Some trace of the former widened 

 condition of the infraorbital canal can still be discerned in many 

 living Sciuro7norpha. Thus a little groove leading up towards 

 the orbit from the edge of the infraorbital foramen seems to 

 represent the closed part of the fissure — the more so since from 

 the floor of the groove two muscles take origin, viz. the dilator 

 narini and a muscle of the upper lip, which in Murides arise 

 from the edges of the upper and outer walls of the wide canal. 



CASTORID^. 



BEAVERS. 



These are medium-sized or large rodents, formerly enjoying 

 a wide distribution in Europe, Asia, and North America. They 

 are known to date from the middle Oligocene of Europe and 

 North America and from the Pliocene in Asia, but they are 

 evidently of still more ancient origin. Represented in the 

 Tertiary by seven or eight genera at least, the group has 

 waned, having as its present sole survivor the genus Castor, or 

 Beavers, now hastening towards extinction. 



In relation to the Sciurides, the Castoridce play a part 

 resembling that of the Microtince towards the Murine. They 

 have followed either a strictly tenestrial or an aquatic mode of 

 living, as opposed to the freer, typically arboreal course 

 pursued by the Sciuridcs. They are more primitive than the 

 latter, in retaining a well-developed thumb, armed with a large 

 claw ; in lacking a bony roof to the orbit ; and in having the 



