EXTERNAL CHARACTERS OF THE BEAVER. 1171 



52. On the External Characters of the Beaver [Castoridce] 

 and of some Squirrels {Sciuridce). By R. I. POCOCK, 

 F.B.S., F.Z.S. 



[Received October 20, 1922 : Read November 21, 1922.] 



(Text-figuves 38-60.) 



Contents. 



Page 

 Introduction 1171 



The Muzzle, Mouth, and Facial Vibrissas 1173 



TiieEar 1177 



The Feet 1183 



«. Feet of the Tropical Asiatic Squirrels 1184 



b. Feet of the Tropical African Squirrels 1188 



c. Feet of the European and American Squirrels 1190 



d. Feet of the Chipping Squirrels 1193 



e. Feet of the Bristly Ground Squirrels 1195 



/". Feet of the Sousliks and Marmots 1198 



^. Feet of the Flying Squirrels (Petauristidfe) 1200 



7i. Feet of the Beaver (Casto") 1205 



The Anus and External Genitalia 1208 



Introdicctio7i. 



The Squii'rels, Marmots, and other admittedly related genera 

 forming the Seiuridte were formerly associated with the ^Beavers, 

 or Castoridas, in a primary division of the Simplicidentate 

 Rodents, the Soiuromorpha *. It has become the fashion in 

 recent years to drop this grouj). Kevertheless, the two families 

 are still suggestively juxtaposed by systematic writers, and no 

 one seems to doubt the validit)^ of tlie claim of their relationship 

 based upon certain cranial characters connected more particularly 

 with the structure of the zj'gomatic arch. Miller, for example, 

 in his key to the families of Rodents of Western Europe, places 

 the Castoridcs and Sciuridee under a heading distinguished from 

 the other families by having the anterior portion of the zygo- 

 matic arch formed chiefly by the jugal bone and the infraorbital 

 foramen small ; and the opening words of his diagnosis of the 

 family Oastoridse are " characters essentially as in the Sciuridse." 

 Alston, Thomas, Flower, Tullberg, Winge, Max Weber, and 

 others all agree in placing the Castorida^ and Sciuridse in the 

 same section of Rodents. 



As regards the division of the Sciuridse into subfamilies there 

 has been, and is still, some difference of opinion. By most 

 authors the Marmots and Sousliks were separated as a subfamily 

 — Arctomyince [Marmotince) — fi'om the Squirrels (Sciurince) ; but 



* Also containing the Aplodontidas, about which I have no first-hand knowledge. 

 This family w^as, however, removed from the Soiuromorpha by Thomas, P. Z. S. 

 1896, p. 1015. 



