THE BEAVER 679 



and flat, its greatest width (about half-way down) equalling about one- 

 third of its length ; at its base it is densely furred, but elsewhere its 

 covering consists of large, flattened scales, between which are scattered 

 a few short, stifif hairs. The urino-genital organs, anal glands, and 

 anus open into a cloaca, so that apart from the mammae, of which the 

 female has four, the sexes look alike externally. 



The pelage consists chiefly of a very dense, long underfur (about 

 25 mm. long on back), with a far scantier growth of long, coarse hairs 

 (50 to 60 mm. long) ; the latter are more abundant on the back, where 

 they nearly conceal the underfur, but towards the flanks and on the 

 underside they are more scattered, and leave the underfur plainly 

 visible. The upper surfaces of the hands and feet are clothed with 

 short hair, which completely conceals the scales. 



The general colour of the head and body is a lighter or darker 

 yellowish-brown, imparted chiefly by the longer hairs : the tint is 

 usually duller below than on the back, and greyer or more pallid about 

 the face and chin ; the hairy parts of the hands and feet are brown ; the 

 naked muzzle pad, soles, palms, and the scaly portion of the tail are 

 dusky. Partly or wholly albinistic specimens are sometimes met with, 

 and their skins have sometimes a beautiful, iridescent lustre. 



In addition to the family peculiarities described above, the strongly- 

 built skull is characterised by the remarkable cellular excavation of 

 the ventral surface of the basioccipital. The auditory bullae are small, 

 and each has a very long, tubular or spoutlike, external meatus. The 

 zygomatic arches are very strong and widely expanded. The anterior 

 palatal foramina are short and narrow ; the tooth-rows are anteriorly 

 convergent, and the pterygoid fossae are deep, the short alisphenoid 

 canals opening into their outer margins. The nasals taper posteriorly 

 and terminate in the interorbital region well behind the ends of the 

 ascending branches of the premaxillas; in the American Beavers the 

 nasals are shorter and terminate posteriorly at the level of the 

 lachrymals. 



The mandible is strong and deep ; its rami are very firmly 

 connected with each other : the strong coronoid processes rise consider- 

 ably above the rounded condyles ; the angular processes are rather 

 small, and their posterior tips rise above the level of the cheek-teeth. 



Dentition: — Both the incisors and the cheek-teeth when unworn 

 show traces of a former, primitive, tubercular or brachyodont structure. 

 The tips of quite unworn incisors are very slender, rounded, and 

 coated on both their anterior and posterior surfaces with enamel ; they 

 show more or less distinct traces of apical tubercles and of grooves, 

 which appear to represent the valleys which originally separated the 

 incisor cusps; such parts are, of course, wholly ephemeral, and as 

 vanishing structures betray a high degree of individual variability. 



