SCIUROMORPHA 



The CastoridcB and SciuridcB, or Beavers and Squirrels, 

 belong to the Sciuromorpha — one of the three great tribes in 

 one or other of which most of the living Simplicidentata can be 

 readily arranged. Besides the two families in question, the 

 Sciuromorpha comprise the American families Heteromyidcs 

 (Kangaroo Rats and Pocket Mice) and Geomyidcs (Gophers). 

 Winge claims the Sciuromorpha as descendants of an ancient 

 rodentian stock, the Aplodontiidce, 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 Sciuromorpha 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 Sciuromorpha 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 Aplodontia the masseter muscles still retain 

 essentially this arrangement. 



VOL. II. 665 2 u 



