1916] Chandler: Structure of Feathers 289 



Barhs 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 (pi. 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 distinctly 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 (pi. 13, 

 fig. Ic) 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 

 (pi. 13, fig. Ih), 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 " buschelf ormig " as in 



ostriches, but with a weakly developed shaft. 



h) 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. 



