HAIR STRUCTURE OF THE MONOTREMATA 47d 
There exists a variety of the shield hair which differs so 
markedly in its appearance from all the other types as to suggest 
at first the necessity of according it a separate classification. This 
aberrant type is restricted to a meager tract about the base of the 
snout, more particularly to an area just ventrad of the base, a 
region which may be termed the chin (fig. 1, A). Figure 8 shows 
the gross appearance of this hair, and figures 32 to 35 its structure. 
All distinctions between shield, isthmus, and shaft have disap- 
peared, but the configuration of the cuticular scales, their mode 
of imbrication, and the character of those dissipated medullary 
cells which still remain are the same as those of all the other 
types of shield hairs. The fact that these ultimate structural ele- 
ments of the hair preserve their integrity of appearance even 
though the general aspect of the hair as a whole may be radically 
modified, would seem to substantiate the assumption that, in 
Ornithorhynchus at least, the cuticular scales and the medullary 
cells offer the readiest means of identification and the most 
trustworthy criterion for the classification of the various types of 
hair. 
The finer hair, found next the skin and underneath the shield 
hair, and which will be termed the ‘fur hair,’ is of a grayish- 
brown color, a trifle darker on the dorsum than on the venter. 
It covers the entire body, with the exception of the tail and feet, 
and is everywhere approximately from one-third to one-half the 
length of the shield hair (fig. 9). Vestiges of this fur hair may 
sometimes be found upon the tail in adults and a considerable 
amount is present on the tail of young individuals. This fur 
hair soon disappears, as a rule, early in adult life over such por- 
tions of the body as the tail and more distal portions of the 
limbs, where its presence is unnecessary for warmth and protec- 
tion from water. Like the shield hair, the fur hair occurs upon 
one restricted portion of the body in a much modified form, 
namely, upon the area immediately surrounding the pinnae of 
the ears. Here it differs from the typical form in both length 
and diameter, though its characteristic histological structures are 
unchanged. It is, as has been said, the softest and finest hair 
which the creature possesses. 
