336 Unwersity of California Publications in Zoology Vou. 18 
tively short, but the vroximals, (pl. 23, figs. 36b, 36c) almost exactly 
as in Buteo. Falco sparverius, again, has a long pennulum on 
the distal barbules. In Haliaeétus leucocephalus the pennulum of 
the distals is short, and the basal dorsal cilia unusually stout 
(pl. 23, fig. 39a). The proximals (pl. 23, fig. 38b) have longer 
and more prominent ventral teeth than in Buteo. Polyborus cheri- 
way agrees with Haliaecétus in the small pennulum and spinelike 
dorsal cilia of the distal barbules (pl. 28, fig. 40a). 
The structure of the feathers of Circus hudsonius was worked 
out in detail by me (1914) ; they agree with Buteo in all important 
details. 
In Gyps fulvus, representing the Vulturidae, the barbules of 
the outer vane of the remiges differ considerably from the type, as 
shown by plate 22, figures 33c and 33d. The distal barbules have 
enormously developed hooklets and ventral cilia, giving the barbule 
a very unique appearance. The proximal barbules on the terminal 
portion of the barbs do not acquire hooked cilia as in other Accip- 
itres, but differ from those of the inner vane merely in the slight 
proliferation of the ventral teeth, which are only partially trans- 
formed into cilia, as shown in plate 22, figure 33d. The back 
feathers of this species differ from those of more typical accipitrines 
only in the great width of the pennulum of the distal barbules 
(pl. 22, figs. 38a, 330). 
c) Down 
The down barbules are very variable, even within a single genus. 
In the majority of the group, e. g., Accipiter cooperi, Pandion 
carolinensis, Circus hudsonius, Buteo borealis, and Gyps fulvus, 
the barbules are elongate and very slender, with very slight swell- 
ings at the nodes, and short prongs toward the tip. In Falco, how- 
ever, the nodes are more enlarged, not conspicuously so in F. ruste- 
cola or F. peregrinus but strikingly so in F. sparverius where the 
pigmentation is in deep nodular spots. As shown in plate 35, figure 
105, the internodes are very slender and frequently wavy, the latter 
condition being very unusual in down barbules. 
d) Relationships 
The microscopic morphology of the feathers of the Accipitres 
presents many points which are difficult of interpretation. Admit- 
ting their relationship with the Cathartae, which seems to be 
