1916] Chandler: Structure of Feathers 301 



the plumage by the wearing away of the barbules on the exposed 

 portions of the barb. 



The breast feathers of the Colymbidae differ very considerably 

 from those of the Gaviidae. In Aechmophorus occidentalis the 

 breast feathers are exceedingly dense, inserted perpendicular to 

 the contour of the body, with the terminal portion sharply turned to 

 lie flat on the contour, this arrangement resulting in an unusually 

 dense plumage. The feathers are peculiar in having the barbs 

 set conspicuously wide apart on the shaft, about 13 per centimeter, 

 and set at rather a wide angle. The barbules are set about 20 per 

 millimeter on each side, and are about 0.75 millimeter in length. 

 They are of a very unusual type, flattened for about half their length 

 and then filamentous, the flattened portion being spirally twisted. 

 There is a further complication in that only every second, sometimes 

 every third, barbule reaches across to the neighboring barb, the 

 intervening ones being twisted so as to lie nearly parallel to the 

 barb and ventral to it (pi. 16, fig. 9/i). On the outer portion 

 of the more distal barbs this peculiar structure is lost and the 

 barbules become elongate (over 1 mm.), slightly flattened, and 

 less twisted, and develop on their distal ends a double series of 

 barbicels, the ventral ones curved and hooklike, exactly similar to 

 the barbicels of the distal barbules in penguins. At the same time 

 the barbs and barbules become ribbed in such a way as to become 

 strongly reflective, and they give a shiny, silky appearance. The 

 result of ^:his peculiar structure is a very much curved, loose, 

 open-vaned feather, which in the aggregate gives the thick silky 

 breast so characteristic of grebes. Various species of Colymhus and 

 Podilymhus show precisely the same structure. 



c) Down 



The down of loons (Gaviidae) very closely resembles that of pen- 

 guins, being very short, usually under 0.5 mm., and with well-devel- 

 oped prongs, exactly as in penguins. In grebes the down is consid- 

 erably longer, often considerably over 1 mm. long, and frequently 

 with the prongs very slightly developed. 



d) Relationships 



As will be seen from the above descriptions, the feathers of 

 grebes and loons are very highly specialized and differentiated, and 



