1916] Chandler: Structure of Feathers 311 
base and pennulum, four very short, stout hooklets, and a tapering 
pennulum with a typical series of ventral cilia, but no dorsal ones. 
On the silvery grey portion the barbules resemble those of the 
similar portion of the secondaries, but are still more transformed. 
The base is shortened and greatly reduced, and the pennulum is 
enormously expanded and inflated (pl. 18, fig. 18e), with short re- 
duced hooklets, and greatly elongated filamentous ventral cilia lying 
in a close, dense brush. The barbules of the back feathers are ex- 
ceedingly dense, the distals bemg set 45 per millimeter, and the 
proximals only about 18 per millimeter. 
The breast feathers of Plotus anhinga are entirely downy, the 
barbules being short on the terminal portion of the feather, but 
long on the basal portion, where they are also very dense, there 
being over 50 per millimeter on each side. 
(2) Fregatidae 
The Fregatidae, containing only the genus Fregata, have the 
barbules of the remiges strikingly similar to those of Phalacocorax, 
but differ in being of enormous size relative to the size of the 
feathers (pl. 18, figs. 15a, 15b). Comparing these figures with figures 
14a and 14b of the same plate, which represent barbules of a 
feather of similar size in Phalacocoraz, the difference is plainly 
evident. The proximal barbules of the imner vane have a_ base 
which is 0.9 mm. in length. In spite of their large size they are 
set very close together, there being about 32 distals and 17 prox- 
imals per millimeter. 
The iridescent feathers of the back have the barbules completely 
transformed for the production of color. The distal barbules of 
the iridescent purplish and greenish-black feathers have short, in- 
conspicuous bases, three or four small moderate hooklets, and flat, 
expanded pennula with constrictions between the cells. This method 
of iridescent color production is exactly similar to that of ducks, 
and the pennula are of precisely the same type as that in Anas 
platyrhynchos (pl. 21, fig. 282). In Fregata the proximal bar- 
bules also share in the iridescent effect by means of the prominent 
dorsal ridges between the cells bearing the ventral teeth, a condi- 
tion frequently found in birds with dark iridescent feathers, e. g., 
Geococcyx (pl. 30, fig. 73a). These proximal barbules of Fregata 
have rather slender, tapering bases, and the ventral teeth tend to 
become separated, to increase in number, and to develop as short. 
