OF THE BEAVER AND OF SOME SQUIRRELS. 1173 



the most part neglected by previous workers, partly owing to 

 scarcity of materials, either fresh or preserved in alcohol, suitable 

 for the purpose. The result of my observations, extending over 

 many years and based upon specimens that died in the Zoological 

 Gardens, form the subject-matter of the present paper. Examples 

 of a good many genera have passed through my hands in that 

 way ; but in some cases I have been compelled to rely upon dried 

 skins contained in the collections of the Zoological Society and 

 of the Natural History Museum *. 



The Muzzle, Mouth, and Facial Vibrissce. 



The muzzle in all the genera of Sciuridse and Petauristidaj 

 is very uniform in structui-e. A peculiarity of the rhinarium is 

 the mechanism by which the nostrils are closed. The hairy skin 

 of the muzzle just behind it on the dorsal side is capable of being- 

 moved downwards and forwards, carrying the supranarial por- 

 tion of the rhinarium with it, the two constituting, as it were, 

 a kind of hood or cap over the nostrils. When completely 

 lowered, the inferior edge of this hood forms a transverse or 

 crescentically curved line defined from the upper lip by a narrow 

 slit of the same shape. The line along which the upper half of 

 the rhinarium is folded down is usually, at all events, marked 

 by a transverse groove just above the upper edge of the nostrils. 

 The I'hinarium itself is naked and has a well-marked median 

 groove between the nostrils, which are of the usual shape, with 

 the typical harial slit behind and externally, and the expanded 

 orifice in front and internally. They are moderately widely 

 separated and look straight forwards. The rhinarium varies 

 somewhat in shape. For instance, in Sciurus vulgaris it is 

 comparatively deep, and has a nearly straight or convexly 

 rounded upper edge. In Sciurus saltuensis it appears to be lower 

 with the upper edge not nearly so raised. It also appears to be 

 broader and altogether larger relatively in Marmota than in 

 Cynomys. But although these observations were made upon 

 fresh specimens, the apparent height of the rhinarium varies so 

 much in accordance with the expansion or contraction of the 

 hood, that I have found it impossible to make any useful syste- 

 matic generalisations with regard to its shape. (Text-figs. 38 & 

 39, A-0.) 



The two halves of the upper lip are completely divided up to 

 the inferior median point of the rhinarium. 



The rhinarium in Castor is very different from that of the 

 Squirrels. The nostrils are widely separated, look outwards and 

 upwards and are valvular, closing to form a crescentic slit. 

 The area around them is naked, and the wide naked internariai 

 space has an oblique crest of hairs passing over the nostrils on 



* I have to thank Mr. Olclfield Thomas not onlj'- for permission to examine the 

 collections under his charge, but also for much kind help in the identification of some 

 of the species of this difficult group about which I was in doubt. 



Proc. Zool. SOC.--1922, No. LXXVIII. 78 



