18 SMITHSONIAN MISCELLANEOUS COLLECTIONS [voL. 48 
ones, but such nearly spineless hook-fibers are not met with in the 
remiges of any other birds. All the other conditions described as 
present in owls, such as the teeth on the outer vane of the first hand- 
remex, the down on the upper surface, the upraising of the terminal 
threads and the overlapping of the layers of the latter, are also met 
with in the remiges of Caprimulgi (figs. 13, 30). Clark (1894, pp. 
569-570) arrived at the result that these two groups agree in being 
aquincubital, a character which he considers of importance, and that 
the number of rows, the arrangement of the coverts and the relative 
position in the hand-remiges is the same in both. On the other hand, 
the number of hand-remiges in the, Striges is always greater than 
in the Caprimulgi. To this we can now add that the Striges and 
Caprimulgi possess peculiar feather structures common to both 
through which they deviate from all other birds. 
The differences between them are the different development of the 
ventral ridge of the secondary quills, which in Striges is high and 
sharp while in Caprimulgi it is short and blunt, and the different 
structure of the terminal threads of the hook-fibers, which in the 
former possess numerous spines, while in the latter they are nearly 
spineless. Common to both are the length and upraising of the 
terminal threads of the hook-fibers, which form the down on the 
dorsal side of the feather, and the comb-like structure of the anterior 
margin of the first hand-remex. 
The hook-fibers spring at an angle of 30 to 40° from the dorsal 
part of the inner side of the secondary quill. All the hook-fibers 
of a secondary quill are parallel to each other and equally far apart. 
It is to be noted that the distance between adjacent hook-fibers is 
nearly the same in all birds, varying only from 20 to 30p. In 
Cypselus this distance is 25 p, in Columba 22p, and in Diomedea 27 p. 
The hook-fibers arising from one secondary quill extend as a rule 
about as far as the next secondary quill, excepting that in the Striges 
and Caprimulgi they extend, as we have seen, much further, al- 
though unusually long hook-fibers occur in some other birds also. 
In Diomedea, for instance, the spined terminal threads of the hook- 
fibers extend to the second secondary quill. The hook-fibers are not 
of equal length in different parts of the wing of the same bird, and 
are different even in the different parts of the same feather. These 
local variations in form and size, met with in all birds, are particu- 
larly striking in Nyctea nivea. In this owl the hook-fibers on the 
broad inner vane are longer than on the outer vane. Considerable 
variations occur also in the vanes of different feathers. I will illus- 
trate this by a few measurements made in the inner broad vane of 
