14 SMITHSONIAN MISCELLANEOUS COLLECTIONS [voL. 48 
continued obliquely downward from here toward the base of the 
fiber. At the end of the proximal band-like portion of the fiber 
the lower halves of these cell limits change their direction and form 
a straight line with the upper halves, which in the distal part of this 
portion of the fiber attains a convexity towards the base (fig. 16, 
Zgr). The transverse curvature is not the same throughout the 
proximal portion of the fiber, being slightest at the base and be- 
coming greater distally. The upper and lower part of the band 
enclose an angle which is very obtuse proximally and becomes more 
acute distally; at the distal end of this portion of the fiber it is 
nearly a right angle (fig. 26). The series of transverse sections 
show that the difference in thickness between the upper, vertical 
and the outwardly lower, nearly horizontal part becomes greater 
distally. The upper vertical and the lower horizontal part of the 
fiber separate distally. <A little below the spot where these separa- 
tions occur the vertical upper portion passes. into the thread-like, 
distal part of the fiber. In ‘Columba livia the lower, horizontal mem- 
branous part terminates in several large lobes narrow at the base 
and broadened distally like leaves (fig. 8, wuLl,). The upper por- 
tions of these lobes stand vertical and enclose with the under ones 
an angle of nearly 90°. Columba livia is the only bird in which I 
have met with such high development of these lobes (fig. 26, VnL,). 
In others the lobes appear as digitate processes, one or two in num- 
ber, turned down distally as in Cypselus, Diomedea, Nyctea, Podar- 
gus (figs. 9, I1, 12, 13, VnL) and others. Each lobe springs from 
a different cell and is to be conceived as a simple cell-diverticulum. 
Hook-shaped extensions hang down from the lower side of the 
distal thread-like portion of the hook-fibers which forms the con- 
tinuation of the upper thickened portion of the basal part. The 
bases of these hooks enfoliate the fiber. The hooks themselves are 
band-like, twisted in their upper part, and terminate in a strong 
backward turned spine (fig. 8, K). While the greater part of the 
band forming the hook is thin its end is thicker and forms a sort of 
swelling which bears the short terminal point, which turns back- 
ward to form the hook. In Columba livia and in numerous other 
birds 4 or 5 hooks occur on each hook-fiber (figs. 8, 10). I found 
fewer hooks in Cypselus (fig. 11), Micropus melba, Macropteryx 
mystaceus, where only 2 to 4 are present, and more in Diomedea 
(fig. 9) where 6 to 8 are to be found on each hook-fiber. It is 
usually relatively broad, rarely long and slender, as in Diomedea, the 
Striges, and Caprimulgi (figs. 9, 12, 13); those nearest the base 
of the fibers are the shortest and are directed vertically downward. 
