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 beige 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 (pl. 16, fig. 9h). 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 hooklke, 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 this 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 Colymbus and 
Podilymbus show precisely the same structure. 
c) Down 
The down of loons (Gaviidae) very closely resembles that of pen- 
euins, 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 shghtly developed. 
d) Relationships 
As will be seen from the above descriptions, the feathers of 
grebes and loons are very highly specialized and differentiated, and 
