1212 EXTERNAL CHARACTEES OP THE BEAVER. 



If the structural differences between the Flying Squirrels and 

 the less specialised true Squirrels entitle these groups to be 

 regarded as distinct families, the differences between the Beavers 

 and the Squiirels must be granted much higher rank. The 

 Sciuridse and Petauristidse may be associated as the Sciuroidea, 

 and the Beavers kept apart as the Castoroidea. The Petauristidse 

 are a compact group represented by several genera and subgenera ; 

 but, so far as my limited observations on the external characters 

 are concei-ned, I have found no reasons for thinking them 

 susceptible of grouping under headings of subfamily rank. The 

 genera of Sciuridse are much more varied ; but their affinities are 

 at present too little understood to warrant the subdivision of the 

 family. 



