20 SMITHSONIAN MISCELLANEOUS COLLECTIONS [voL. 48 
only ventral spines ; these ventral spines are, however, extraordinarily 
long. In the terminal threads of the hook-fibers arising toward the 
middle of the secondary quill, spines appear on the dorsal side also. 
Where dorsal spines are developed the ventral ones are shorter than 
elsewhere. Below the middle of the secondary quill the dorsal and 
ventral spines are equal and about half as long as the ventral spines 
of parts where no dorsals are developed. The down becomes grad- 
ually thicker outwards and is thickest and highest beyond the middle 
of the feather-vane, measuring transversely, just where the termi- 
nal threads of the hook-fibers and the spines attain their greatest 
length (fig. 32). In some birds the proximal, band-shaped part of 
the hook-fiber is remarkably strong when compared with the termi- 
nal thread. In Cypselus the proximal portion is actually longer 
than in Podargus, the hook-fibers of which have a much greater 
total length than those of Cypselus. 
B. The Curved Fibers 
Opposite the hook-fibers and a little lower down there rises from 
the side of each secondary quill a second system of fibers, the curved 
fibers (figs. 3, 4, Bgf). At first they extend obliquely outward at 
an angle of 35 to 40°. At about half their length they suddenly 
bend toward the secondary quill from which they arise in such a 
way that their distal portion comes to lie parallel to the latter (fig. 
21). Although many anatomical characteristics indicate that the 
curved fibers and the hook-fibers are homologous, yet these two 
kinds of fibers are in many respects morphologically and func- 
tionally so different that a special terminology and a separate de- 
scription are necessary. 
In the curved fibers as in the hook-fibers two parts, a broader, 
proximal, band-shaped portion and a slender terminal thread can 
be distinguished (figs. 9, 28). 
The fiber increases in breadth from its point of origin up to the 
middle of its length and then narrows again. In sections vertical 
to the secondary quill one can see (figs. 3, 4, 27), that the proximal 
ends of the curved fibers are band-shaped with a thickened ventral 
and a thin dorsal margin. Where the fiber leaves the longitudinal 
ledge on which its basis rests, the two margins become equal in 
thickness. Further on, the dorsal margin remains stout while the 
ventral one becomes thin, membranous and sharp. So far the curved 
fibers resemble the hook-fibers, but a difference between them can 
already be noticed here. In the proximal part of the curved fibers 
the strip forming the upper margin is not merely thickened, as in 
