474. LEON AUGUSTUS HAUSMAN 
Covering the venter is a type of the shield hair somewhat simi- 
lar to that which clothes the head, but bearing a much longer 
shield, and, as might be expected, containing less pigment (fig. 
1, D). Upon the sides of the venter (the region of the transition 
in color from the dark brown of the back to the creamy white of 
the abdomen) the pigment is confined to the proximal fourth of 
the shield. In all cases the shaft of this venter hair contains so 
few of the brown pigment granules as to show no color except 
under the microscope. 
From the middle of the back to the base of the tail the shield 
undergoes a progressive increase in length and rigidity, and the 
shaft becomes correspondingly shorter (fig. 5). When the tail 
is reached each hair consists almost wholly of the flattened, stif- 
fened shield element, a construction which gives it the character 
of very fine bristles. Near the base of the tail the top of the 
shield is weakly pigmented, while near the end of the tail the 
shields of the hairs exhibit an increased number of pigment 
granules. 
The tail hairs are similar to those of the feet, and are subject 
to much attrition. They are consequently considerably worn 
and frayed, and are held closely against the skin. ‘The fore- 
feet are almost entirely shorn of their hair as far as the wrist, 
since it is upon these feet that the brunt of the labor of swimming 
and of burrowing falls. Upon the hind feet more hair is present. 
On both feet the hair is of a light brown color. Unlike the shields 
of the hair upon the venter, the shields of the hair of the feet are 
pigmented with their characteristic color throughout their 
lengths. This suggests that the lighter color of the hair upon 
the fore feet is not due to the fact that the tips of the shields have 
been worn away. 
In all these varieties of the shield hair the three elements, 
shield, isthmus, and shaft, are invariably present (fig. 14, B, C, 
D), though different elements rise into prominence and determine 
the gross appearance of the hair upon different parts of the body 
(figs. 3 to 7). In all of these varieties, likewise, the relationships 
between the cortex, medulla, and cuticle are fundamentall ; the 
same, but in certain of the hairs, as will be shown, the medulla 
is lacking altogether. 
