476 LEON AUGUSTUS HAUSMAN 
The medulla of the fur hair is somewhat similar to that of the 
shield hair (fig. 42), its principal differences lying in the confor- 
mation of the individual cells and in their closer compaction 
in the shaft. The shield hair, however, possesses a structurally 
distinctly different medulla in the region of the shield (figs. 23, 
24, and 42). 
The hairless areas of the body are the ventral surface of the 
tail and the distal portions of the feét. The denuded state of 
the former is acquired by the wearing away of the few hairs 
which make their appearance in the young. Stumps of these 
early haus remain imbedded in the follicles throughout the life 
of the animal and apparently continue to grow, but are con- 
stantly worn away. A similar condition obtains on the distal 
portions of the feet. 
The cuticular scales of the shield hair. At the base of the shield 
hair shaft the exposed portion of the cuticular scales is triangular 
in form, with the acute apex directed distad (fig. 15). As the 
middle of the shaft is approached, the scales become more obtuse, 
until their edges form a series of fine, transverse, roughly parallel 
ridges extending obliquely across the shaft (fig. 13). No differ- 
ence in the thickness of the cuticle of the extal and ental surfaces 
of the cylindrical shaft are apparent (fig. 20). 
The scales of the isthmus (fig. 14, C), are like those of the shaft 
in outline, but are smaller and more compactly grouped. The 
transverse markings indicating their edges are, in consequence, 
more crowded. Due to the angle at which the isthmus is bent, 
the scales in the convex region of the bend are much more closely 
impacted than those above and often present the appearance ‘of 
having been fused into a solid plate. This is due to the fact that 
the edges are so closely crowded together that they are not dis- 
tinguishable except under very careful examination. - Upon the 
outer, or convex side of the bend, where the wear occurs, the 
scale edges are much modified in outline and present a much 
more evenly parallel appearance than any of the others. From 
this and from similar observations on other hairs in various situ- 
ations upon the bodies both of Ornithorhynchus and of other 
species of mammals, it is concluded that the change in form be- 
