6 SMITHSONIAN MISCELLANEOUS COLLECTIONS [voL. 48 
polyhedral and overlap each other like roof slates. Here and there 
a more irregular polyhedral cell is interposed between the others. 
The transverse sections show that these quills are not simple bands 
but curved more or less transversely, convex on one side and con- 
cave on the other. The manner in which the secondary quills, which 
are highest at their proximal end, become lower toward their distal 
end is correlated in many birds to that curvature in such a way that 
the curvature is greatest where the quills are highest and decreases 
with the height outward, finally disappearing almost entirely near 
the distal end. Strong (1902, pp. 158-159), draws attention to an 
extension of the lower margin of the ‘secondary quills which he 
terms “the ventral ridge,” and which he represents as a constant 
character. In early stages of the development of the feather this 
ridge is said to be very large, being much reduced later. This ven- 
tral ridge of the secondary quills, which also appears in the covert 
feathers of the wing (fig. 5), is functionally a very important char- 
acter of all remiges. It is small in Columba, Cypselus and others, 
larger in the Caprimulgi (figs. 2, 4, 7, Hnl), still further developed 
in the owls (fig. 29, Hnl) and in many water birds, and largest in 
Aquila and Diomedea. Hand in hand with the development of this 
ventral ridge goes the development of the curvature. The great 
development of the ventral ridge, combined with the strong curva- 
ture of the secondary quills in Aquila and Diomedea probably enables 
these birds to perform the sailing flight characteristic of them. It 
must be noted, however, that the ventral ridge is very well developed 
in owls and various water birds also, which are not such excellent 
sailors of the air as the albatross and the eagle. Ahlborn (18606, 
». 20) describes a peculiarity of certain parts of the remiges in the 
duck, the swan and other birds. He says that portions of the lower 
sides of these feathers seem to be covered by a fine membrane, this 
appearance being due to the fact that here the secondary quills are 
not only connected with each other dorsally by the tertiary fibers 
but also ventrally by delicate membranous extensions of their ventral 
margins, which bridge over the spaces between them. These ex- 
tensions enclose a right angle with the band shaped secondary quills 
from which they arise and exactly fit on the ventral margins of the 
quill in front, to which they adhere on account of their intrinsic 
elasticity. This membranous extension of Ahlborn .is only the 
strongly developed ventral ridge of the secondary quills. 
The decrease outwardly in the height of the secondary quills is 
uniform in some birds but not so in others. In the latter there is 
a sudden decrease in height in the secondaries of the broad inner 
