1916] Chandler: Structure of Feathers 289 
Barbs very similar to those of ostriches, with no perceptible 
ventral or dorsal ridge, and with barbules attached almost at right 
angles in an even series almost directly opposite each other, and 
not at obviously different levels as in pennaceous feathers. Change 
in number of barbs per unit of measure from base to tip of feathers 
considerable; about 20 per centimeter at base of feather, dimin- 
ishing to only 8 or 9 near tip. 
Inner and outer vanes similar. Barbules considerably advanced 
over those of ostriches in their greater variability in different 
feathers and parts of feathers, also in their closer approximation to 
both a pennaceous and downy type. Set about 35 per millimeter on 
each side on basal barbs, and only about 20 per millimeter on terminal 
ones. No differentiation between distal and proximal barbules. The 
best developed barbules on basal portion of barbs on terminal halves 
or remiges (pl. 13, fig. la). Length about 2 mm., the basal one- 
third, more or less, considerably broadened into specialized base, not 
bilaterally symmetrical, but furnished with a series of ventral 
prongs or barbicels, the dorsal edge smooth and unbroken. Pen- 
nulum cylindrical, with more or less well-developed prongs for its 
entire length. Development of basal portion into a differentiated 
unsymmetrical base, with distinetly barbicel-like prongs, and of 
terminal portion into a filamentous pennulum, shows distinct approxi- 
mation to pennaceous barbules, at least much nearer than the simple, 
ribbon-like barbules of ostriches. 
Barbules from middle part of either vane of same feather (pl. 13, 
fig. lc) considerably less specialized. Basal portion much narrower 
with less distinct barbicels, and a much less obvious distinction be- 
tween flattened base and filamentous pennulum. 
(2) Other Feathers 
No essential differences from remiges in structure of body feathers, 
but barbs more numerous. On small rump feather they decrease 
from 40 per centimeter on each side at base to about 22 at tip, in 
upper back feather less numerous, 30 per centimeter basally to about 
13 at tip. In these cases number of barbs per unit of measure 
apparently increases inversely to size of feather, or, in other words, 
space between barbs is directly proportional to size of feather. 
Barbules less differentiated into base and pennulum than in remiges 
(pl. 18, fig. 1b), and shorter, with decided tendency toward ordinary 
downy type, similar to that of penguins, and to neossoptiles of 
many water-birds. 
I have had no opportunity to study neossoptiles of rheas, but 
they are stated by Gadow (1891) to be ‘‘buschelformig’’ as in 
ostriches, but with a weakly developed shaft. 
b) Relationships 
Like the Struthioniformes, the rheas show characters which 
might be construed as evidence of their being primitive in their 
flightless condition and of not being descendants of flying birds. 
