1916] Chandler: Structure of Feathers 311 



base and pennulum, four very short, stout booklets, 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 (pi. 18, fig. 13e), with short re- 

 duced booklets, and greatly elongated filamentous ventral cilia lying 

 in a close, dense brush. The barbules of the back feathers are ex- 

 ceedingly dense, the distals being 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 (pi. 18, figs. 15a, 15&). Comparing these figures with figures 

 14a and 14& of the same plate, which represent barbules of a 

 feather of similar size in Phalacocorax, the difference is plainly 

 evident. The proximal barbules of the inner 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 booklets, 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 (pi. 21, fig. 28^). 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 (pi. 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- 



