16 SMITHSONIAN MISCELLANEOUS COLLECTIONS [voL. 48 
microphotographs of the surface of a wing feather of Cygnus olor 
one sees the strongly developed dorsal spines very plainly (fig. 23, 
W,). Surface views show that each hook-fiber is curved in a 
horizontal plane, the convexity being turned towards the secondary 
quill from-which it arises. This curvature is confined to the central 
part of the fiber. The proximal and distal parts of the fiber remain 
straight and lie in the same straight line (figs. 17, 18, 19, 22). The 
upper, thickened margin, which is vertical in the proximal part of 
the fiber, becomes oblique in the neighborhood of the hooks, in- 
clining above toward the secondary quill; distally it resumes its 
former vertical position. A similar twist, more or less pronounced, 
is observed in the hook fibers of all birds with the exception of the 
owls and Caprimulgi, where the upper margin of the fiber is vertical 
throughout its whole length. 
The ventral part of the proximal band-like portion of the fiber, 
which terminates in the processes of the ventral membrane, is in 
Columba about as long as the distal portion; in smaller birds like 
Fringilla and also in the Psittaci, the proximal part is longer than 
the distal; in the Striges and Caprimulgi the distal part greatly ex- 
ceeds the proximal in length (figs. 12, 13), this being one of the 
differences between the remiges of the diurnal and nocturnal birds. 
The most striking peculiarity of the remiges of the owls is the 
down covering the dorsal side particularly of the proximal part 
of the broad inner vane, which gives them a velvety appearance. 
This velvet-like down consists of the lengthened distal portions of 
the hook-fibers. Fatio (1886, p. 257) says that they are long, slen- 
der and covered with numerous lateral spines (barbicels), which 
on account of their length and irregular arrangement make the fiber 
appear like a down feather. The proximal band-shaped part of 
the hook-fibers of the remiges of owls (fig. 12) has the same struc- 
ture as in other birds. The terminal processes of the ventral mem- 
brane are usually one, less frequently two in number. They are 
small, narrow and show a slight hook-shaped curvature. In the 
region of the long and thin fourth and fifth hooks, the fiber is very 
narrow, and here the cell limits, usually indistinct, are sometimes 
conspicuous enough. The most remarkable feature of the remiges 
of owls is the extraordinary development of the distal, thread-shaped 
part of their hook-fibers. These terminal threads consist, according 
to the position of the fibers, of a row of 10 to 50 cells, each one 
bearing one or two peculiar, long and very thin spines (fig. 12, W). 
The dorsal spines are not raised but lie horizontal, so that they point 
inward toward the primary quill of the feather. The ventral spines 
