258 University of California Publications in Zoology fVoL. 13 



when both present, are nearly equivalent in size and are practically 

 indistin^ishable 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 structui'e 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 signiflcant 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 

 "powder-down", a description of which was given by me (1914) in 

 the case of Circus hudsonius. Powder-down, according to Gadow 

 (1891), occurs in the following birds: All Ardeae, Balaeniceps, 

 Bhinochetus, Eurypyga, Mesiies, Tinamidae, a few Falconiformes, 

 some Psittaei, Podargus, Leptosoma, Coracias, and only one passerine 

 genus, Artamus. I have also found it in the burrowing owl, Speotyfo 

 cunic'ularia. 



Although sometimes found scattered promiscuously^ among the 

 plumules, as in Gypaetus 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. 



