BILL AND HAIRS OF ORNITHORHYNCHUS PARADOXUS. 161 
is seen to be more developed on the under side, and a broad un- 
pigmented margin is always found along the upper surface. 
This is in part due to the cuticle of the upper surface, which 
is much thicker than that of the under, as can be well seen in 
sections (fig. 18). The degree to which the pigment extends 
towards the upper surface differs, however, in individual hairs. 
Passing along the centre of the shield is a very distinct medulla 
containing abundant gas-bubbles, entering as a result of the 
contraction of soft protoplasmic cells, which are present and 
stain in logwood, &c., in the shields which have not yet 
appeared above the surface (fig. 18). Traces of this medulla 
can be followed nearly to the tip of the structure. The pigment 
is sometimes so far restricted to the under side of the shield as 
to be entirely beneath the level of the medulla in the middle 
line, but it extends further at the sides (fig. 18). 
Below the shield there is a constricted neck in which the 
medulla is apparently wanting and the pigment scanty, but 
immediately below, at the beginning of the long shaft, the me- 
dulla is strongly developed suddenly and the pigment assumes 
the characteristic ladder-like appearance, being arranged in 
bands alternating with spaces containing colourless shrunken 
cells and gas bubbles. The arrangement is not, however, so 
regular as that of the corresponding part of the smaller hairs. 
Below, the shaft passes into the somewhat more slender unpig- 
mented base in which a medulla is wanting. 
Hence in one of the larger hairs we can distinguish a shield, 
neck, shaft, and base. 
The small hairs can be divided into similar regions. The 
free tip 1s unpigmented, and is followed by a section corre- 
sponding to the shield, with the same longitudinal arrange- 
ment of small granules and large fusiform masses of pigment. 
Traces of a medulla marked by gas bubbles are to be seen in 
places. Then follows the part which represents the neck,—not, 
indeed, more slender, but with the same diminution in the 
amount of pigment and cessation of the medulla. To this 
succeeds the shaft with an extremely uniform alternation of 
medullary dark and light bands, the latter containing distinct 
