666 CASTORIDZ: 
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 mediais; 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 Sczwromorpha. 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 dzdator 
narvint and a muscle of the upper lip, which in J/urzde@ arise 
from the edges of the upper and outer walls of the wide canal. 
CASTORIDZ. 
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 Cas/or, or 
Beavers, now hastening towards extinction. 
In relation to the Sczurzde, the Castorirde play a part 
resembling that of the MWicrotine towards the Murine. They 
have followed either a strictly terrestrial or an aquatic mode of 
living, as opposed to the freer, typically arboreal course 
pursued by the Sczur?de. 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 
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