258 University of California Publications in Zoology (Vou. 18 
when both present, are nearly equivalent in size and are practically 
indistinguishable from each other either by structure or position. 
The rami of plumules are extremely long, slender, filamentous, 
and very numerous, the cellular structure not apparent, and with 
no edges or dorsal or ventral ridges. The barbules are always of 
downy type, and have the same structure as the downy barbules 
of the contour feathers, being of the type found on the aftershaft 
when this differs from the downy portion of the feather plate, as in 
gallinaceous birds. As a rule the downy barbules of the plumules 
are longer and more numerous than those of the contour feathers 
of the same species, but the difference is often inappreciable. In 
some genera, e. g., in Circus, the barbules of the plumule are more 
slender than those of the down of contour feathers, and more flexible. 
The result of the close setting and great length of both rami and 
barbules, and of their slender, flexible nature, characteristic of most 
plumules, is a very dense, cottony structure which eminently serves 
its purpose as a water-proof, heat-insulating covering for the body, 
as pointed out by me (1914). It is significant that plumules, almost 
without exception, reach a high development and have a uniform 
distribution in all water birds. 
2. Powder-down 
Occurring as a frequent modification of plumules is the so-called 
‘‘nowder-down’’, a description of which was given by me (1914) in 
the case of Circus hudsonius. Powder-down, according to Gadow 
(1891), oceurs in the following birds: All Ardeae, Balaeniceps, 
Rhinochetus, Eurypyga, Mesites, Tinamidae, a few Falconiformes, 
some Psittaci, Podargus, Leptosoma, Coracias, and only one passerine 
genus, Artamus. I have also found it in the burrowing owl, Speotyto 
cunicularia. 
Although sometimes found scattered promiscuously among the 
plumules, as in Gypaétus and many parrots, powder-down is usually 
found in more or less well-defined patches. As Gadow (1891) sug- 
gests, the occurrence of powder-down in such diverse groups of birds, 
and the wide variety of situations in which it is found on individuals, 
make it appear that typical plumules, at different times and in 
different groups, may be modified into powder-down, as the result 
of some unknown stimulation. 
