270 Unwersity of California Publications in Zoology Vor. 18 
than that of the down of the feather proper, and differs in not 
possessing certain specialized characteristics, such as the detachable 
rings at the nodes found in the more basal barbules of gallinaceous 
birds (pl. 36, fig. 108), and the fimbriae which characterize the base 
of the inner barbules in passerine birds (pl. 37, figs. 114 and 115). 
b) Down.—In all but especially modified contour feathers, a vary- 
ing proportion of the feather plate is downy, the transition from the 
downy to the pennaceous portion being sometimes gradual, but more 
frequently abrupt, as shown in text—figure A, the line of demarcation 
varying a great deal in different feathers. In the lower belly feathers 
and tail coverts of some birds, e. g., Leptoptilus, the downy struc- 
ture pervades the entire feather, producing the ‘‘semiplumes’’ of 
Nitzsch (1867) and others. In Leptoptilus these are the feathers 
from which the true ‘‘ 
very similar to these are developed in the lower belly region of 
turkeys, these being employed very extensively as a substitute for 
‘‘marabou’’. 
The minute structure of the down varies to a large extent in 
different groups of birds, the nodes and internodes both displaying 
peculiarities which are highly characteristic of different groups 
(pls. 34-87). 
The structure of down often varies a great deal in a single 
feather, the specific characteristics being always best displayed by 
barbules on the inner portion of the distal vanule of the basal barbs. 
Farther distad on either feather or barb, and on the proximal vanules, 
the structure is often less specialized, and lacks some of the charac- 
teristic features of the group. This is well displayed in the down 
of a turkey, where only the inner portion of the distal vanules of 
barbs on the basal part of the plate possess the peculiar, charac- 
teristic, detachable rings at the nodes (pl. 36, fig. 108). The strue- 
ture of the proximal vanules of the same region of the feather often 
approximates that of the distal vanules, but is never quite so per- 
fect. The outer barbules of all the barbs, and all the barbules of 
the more distal barbs, lose the specialization, becoming finally fila- 
mentous with the nodes very inconspicuous. Though this is the 
order of reduction of specialization where such reduction takes place, 
there are some birds in which the downy structure, though highly 
specialized, is almost uniform, becoming reduced only at the tips of 
the barbs, as in the Anatidae. As stated above, the structure of 
the down of aftershafts and plumules, but not of neossoptiles, is 
9 
marabou’’ of commerce is derived. Feathers 
