334 Vniversitij of California Publications in Zoology [Vol. 11 



holding together several barbs or barbules. Probably just as 

 much of this powder is produced with the part of the feather 

 first formed as with the later part, but due to the spreading out 

 of the feather, and the using up of the powder, it has a less dense 

 appearance. 



In powder-down feathers the barbs are attached more or less 

 evenly entirely around the umbilicus, thus differing from the 

 ordinary down feathers, which always have a distinct though 

 short shaft on either side. 



Oil-Gland Feathers 

 A further modification of the ordinary down feathers is to be 

 found in the "tuft" of the oil gland. On close examination 

 this tuft is found to consist of a single circlet of feathers, four- 

 teen in number in all three of the specimens in which they were 

 counted. The calamus of these feathers, which is about one milli- 

 meter long, is entirely imbedded in the skin. The majority of 

 the barbs, about twenty to twenty-five, spring from the umbilicus 

 in small groups of two to four, i.e., the calamus breaks up into a 

 number of short roots, which almost immediately break into from 

 two to four barbs. There is developed, however, on one side of 

 the quill a single shaft which is very short and rudimentary', 

 but nevertheless a constant feature. The barbs reach a length 

 of approximately one centimeter or less, and bear a series of 

 short, undifferentiated, awl-shaped barbules, averaging 0.54 mil- 

 limeter in length, and inserted about fifteen per millimeter on 

 each side. These barbules, differing so greatly in length from 

 ordinary down barbules, differ further in that they lie quite 

 closely appressed to the barbs, instead of spreading. The func- 

 tion of this circlet of small feathers, which is lacking in many 

 groups of birds and is so characteristic of others, is quite 

 unknown at the present time. 



FILOPLUMES 

 Filoplumes, though nowhere overdeveloped or conspicuous, 

 are found in considerable numbers at the base of the contour 

 feathers of the dorsal, lumbar, and caudal tracts. These peculiar 



