SCIUROMORPHA 
The Castortide and Sciuride, or Beavers and Squirrels, 
belong to the ScrturomorpHA—one of the three great tribes in 
one or other of which most of the living Szmplicidentata can be 
readily arranged. Besides the two families in question, the 
Sciuromorpha comprise the American families /eteromyzde 
(Kangaroo Rats and Pocket Mice) and Geomyzade (Gophers). 
Winge claims the Sczuromorpfha as descendants of an ancient 
rodentian stock, the Aplodontizde, of which the only living and 
doubtless much modified remnant is the remarkable genus 
Aplodontia, comprising the Sewellels or Mountain Beavers of 
the Rocky Mountain region. 
The leading character of the Sczuvomorpha is to be found 
in the skull, in which the infraorbital canal is always small, 
serving only for the passage of the infraorbital nerve and 
accompanying blood-vessels, and transmitting no part of the 
masseter muscle. In this feature the sciuromorphine skull 
departs widely from that of other rodents (in which the canal 
is large, lodging or transmitting a larger or smaller portion of 
the masseter medialis muscle), and resembles the skull of the 
majority of non-rodentian mammals. Nevertheless, according 
to Winge, the Sczwromorpha are descended from ancestors 
possessing, like other rodents, spacious infraorbital canals. In 
these ancestors the masseter medialis muscle had its normal 
rodentian strength and development, and part of it had its 
origin within the infraorbital canal; on the other hand, the 
deep portion of the masseter lateralis, arising on each side from 
the outer and fore part of the zygomatic arch, was not 
unusually large or powerful, and had not yet extended its area 
of origin above the level of the infraorbital foramen. In the 
living genus Af/odontia the masseter muscles still retain 
essentially this arrangement. 
VOL. I. 865 2U 
