310 Unversity of California Publications in Zoology {Vou. 18 
all of the cells clearly marked off by ridges. The barbules are set 
fairly close together, the distals being about 30 and the proximals 
18 per millimeter. 
In the outer vane of the secondaries are to be found the most 
unusual types of barbules in the whole avian class. The portion 
of the vane which possesses a beautiful silvery grey color owes this 
entirely to the pennula of the distal barbules (pl. 18, fig. 183c). The 
bases are similar to those of the inner vane, but the pennula are 
profoundly transformed into thick, clumsy, inflated, sacklike ex- 
pansions, filled with opaque air bubbles which, when the barbules 
are immersed in balsam, become infiltrated and rendered transparent, 
leaving the round nuclei appearing like eyelike spots. There are 
no dorsal cilia whatever, the hooklets are only three or four in num- 
ber, short and heavy, and the ventral cilia are produced into extremely 
long, filamentous processes, lying closely appressed to each other, and 
extending far beyond the tip of the expanded portion of the pennulum. 
There are nine or ten short cells in the pennulum beyond the 
hooklet region, each with a long ventral barbicel, so that there is 
a dense brush of these. The deep black pigment of these barbules 
has a peculiar distribution, beimg dense in the base and in the 
hooklet cells and first two cilia cells of the pennulum, but absent 
in the terminal part of the pennulum. Distal to the silvery area, 
the pennula lose their inflated form and long cilia, then resembling 
those of the inner vane, but with no dorsal spinelike cilia. The 
proximals of the outer vane (pl. 18, fig. 18d) are hardly distin- 
euishable from those of the inner vane, except that the pennulum 
is slightly shorter, and the recurved dorsal spines more prominent. 
The back feathers of Plotus are modifications of the same type. 
The proximals (pl. 18, fig. 18g) have a similar short, relatively 
broad base, and the pennulum with recurved spines, but it is pro- 
duced into a long, slender filament, and ultimately the whole bar- 
bule is transformed into down on the more basal barbs. On the 
more distal barbs the pennula become elongated and lose their 
broad character, at the same time developing a few very weak and 
minute ventral cilia, but the typical form of the whole barbule is 
then soon lost and it becomes merely rodlike in form with a few 
ventral barbicels. The distal barbules also ultimately assume this 
form at the tip of the barbs. On the black portion of the feather 
the distal barbules (pl. 18, fig. 18f) have long, slender bases with 
small but typical ventral teeth, no hump on dorsal contour between 
