260 Mr W. Macgillivray on the Covering of Birds. 



from beneath upwards than in any other direction ; pulled to 

 either side also, they feel stronger than when bent downward in 

 the direction of their concavity. They are elastic, and this pro- 

 perty, together with their curvature, tends to keep them close 

 together, and enables the bird to present, when occasion requires, 

 a more or less compact surface to the air. When the barbules 

 are disjoined,]they readily unite again, on being placed in appo- 

 sition. The weaker the feather is, provided it be complete in 

 all its parts, the greater is the cohesion between its barbs. Com- 

 pare, for example, the quills of Diomedea exulans with those of 

 Falco rufus ; or quill-feathers in general with ordinary feathers. 



The webs ordinarily consist of united barbs, more or less stiff, 

 although elastic, and. compact, in their whole length, excepting 

 toward the junction of the shaft with the tube, where they are 

 of a looser texture, often entirely disunited and floating. The 

 lateral lines, from which the barbs arise, incline toward the me- 

 dian line of the shaft at this place, as has already been explained, 

 and meet at its commencement. 



At this point there is, in the feathers of a large portion of 

 birds, a plumiform process, or small feather, which is of the 

 following description : From the fore part of the tube, at the 

 commencement of the shaft, and lying over the aperture by 

 which the internal membrane of the tube escapes, rises a thin 

 lamina, being a continuation of the substance of the tube. It 

 gradually narrows, and is continued in the form of a very deli- 

 cate thread, for a greater or less extent. From the sides of this 

 shaft rise two series of barbs, and from the barbs two series of 

 barbules, as in the ordinary feather itself, all the parts being 

 extremely fine, and entirely disunited. The barbules are very 

 much elongated and loose, resembling in these respects those of 

 the lower part of the webs of feathers in general. This minia- 

 ture feather may be called the accessory feather^ pluma acces- 

 sorial la plume accessoire. In feathers possessed of this struc- 

 ture, the internal membrane of the tube comes out entire be- 

 tween the accessory feather and the feather properly so called, 

 and is not continued internally along the back of the shaft. 



In respect to this accessory plumule, there is a curious and 

 beautiful gradation among birds. In the diving aquatic birds, 

 or such as swim more than fly, there is a short laminar or squa- 



