191-t] ('handler: Ffnfhcrs of Circus IhkIsoiuhs 349 



use of a single strengthened feather as a cutting edge, and on 

 the shoulder region by the use of numerous very small curved 

 feathers, making a smooth, rounded edge. In the region of the 

 alula the same result is obtained in a different way, namely, by 

 a very sharp curve in the outer vane of the outer thumb feather 

 so that this fits over the edge of the wing like the ridge boards 

 on a roof. 



The four alula feathers lie one above the other, rather than 

 in a liorizontal series, so that the outer vanes of all four come to 

 lie on the edge of the wing, and are curved as is that of the 

 first one. These feathers are further distinguished by the groove 

 on the under surface of the shaft swinging outwards, so that 

 its position is nearer the outer vane. 



The minute structure of the outer vane is somewhat different 

 fi-om that of the other feathers thus far considered. The liarbs, 

 of which there are eighteen per centimeter on each side, bear 

 very short distal barbules (pi. 18, fig. 8), which are from 0.5 to 

 0.6 millimeter in total length. The tips are stout, with very 

 long booklets and with barbicels on one side only, thus present- 

 ing an appearance very similar to that of the distal barbules of 

 the outer vane of the remiges, bixt with the short, stout foi'm 

 and length of the barbicels exaggerated. The proximal barbules 

 (pi. 18, fig. 7), twenty per millimeter, are nearer to the type 

 with barbicels hitherto described than to the normal type, but 

 are neverthelass somewhat different. The upper recurved edge 

 is provided with a series of short, blunt teeth, while the lower 

 edge, for the middle third, is furnished with lobes or barbicels 

 of various grades of development. There is no distinct bend, 

 the base gradually tapering to the tip. The inner vane has no 

 structural peculiarities; it has twenty-two barbs per centimeter. 



The peculiar modification of the outer vanes of the alula 

 feathers to serve a unique function, namely, that of forming a 

 smooth, rounded edge for the wing in this region, is a very 

 interesting adaptation. As has been shown, not only is the 

 macroscopic form of the feather changed by modifications of the 

 barbs and shaft, but there is also a very high specialization of 

 the microscopic structure to produce the strong resistant vane 

 required to adequately serve its peculiar function. 



