EXTERNAL PARTS OF BIRDS. — FEATHERS. 
85 
like processes, while hamuli are hooked at the end ; they are not found on all feathers, nor on 
all parts of some feathers. Barbicels occur on both anterior and posterior rows of barbules, 
though rarely on the latter ; booklets are confined to any an- 
terior series of barbules, which, as we have seen, overlie the 
posterior rows, forming a diagonal mesh-work. The design 
of this beautiful structure is evident; the barbules are inter- 
locked, and the whole made a web ; for each booklet of one 
barbule catches hold of a barbule from the next barb in front, 
any barbule thus holding on to as many of the barbules of the 
next barb as it has booklets ; while, to facilitate this interlock- 
ing, the barbules have a thickened upper edge of the right size 
for the booklets to grasp. The arrangement is shown in fig. 
22, where a, a, a, a, are four barbs in transverse section, viewed 
from the cut surfaces, with their anterior, &, 6, 6, 6, and pos- 
terior, c, c, c, c, barbules, the former bearing the booklets 
which catch over the edge of the latter. 
Fig. 21.— 
A single bar- 
bule, bearing 
barbicels and 
hooklets ; mag- 
nified ; after 
Nitzsch. 
Fig. 
Four 
cross section, a, 
a, a, a, bearing 
anterior, 6, b, h, 
h, and posterior 
c, c, c, c, bar- 
bules, the form- 
er bearing hook- 
lets which catch 
over the latter; 
magnified; after 
Nitzsch. 
Types of Feathery Structure. — But all feathers do not 
answer the above description. The after-shaft may be wanting, 
as we have seen. Hooklets may not be developed, as frequently 
happens. Barbicels may be few or entirely wanting. Barb- 
ules may be similarly deficient, or so defective as to be only 
recognized by their position and relations. Even barbs them- 
selves may be few or lacking on one side of the shaft, or on 
both sides, as in certain bristly or hair-like styles of feathers. Consideration of 
these and other modifications of feather-structure has led to the recognition of 
three types or plans: 1. The perfectly feathery, pZwmows, or pennaceous (Lat. pluma, a plume, 
OT penna, a feather fit for writing with; fig. 23), as above described. 2. The downy or 
plumulaceous (Lat. plumula, a little plume, a down-feather), when the stem is short and 
weak, with soft rhachis and barbs, with long slender thready barbules, little knotty dilata- 
tions in place of barbi- 
cels, and no hooklets. 
3. The hairy, bristly, or 
• filo - plumaceous (Lat. 
filum, a thread), with 
a very long, slender stem, 
Und rudimentary or very 
smaU vanes composed of fine cylindrical barbs and barbules, if any, and no barbicels, knots, 
or hooklets. There is no abrupt definition between these types of structure ; in fact, the same 
feather may be constructed on more than one of these plans, as in fig. 19, partly pennaceous, 
partly plumulaceous. All feathers are built upon one or another, or some combination, or 
modification, of these types ; and, in all their endless diversity, may be reduced to four or five 
Fig. 23. — A feather from the tail of a kingbird, Tyrannus carohnensis, 
almost entirely pennaceous ; no after-shaft. From nature, by Coues. 
Different Kinds of Feathers. — 1. Contour -feathers, pennce or plumcB proper, have a 
perfect stem composed of calamus and rhachis, with vanes of pennaceous structure, at least 
in part, usually plumulaceous toward the base. These form the great bulk of the surface- 
plumage exposed to light ; their beautiful tints give the bird's colors ; they are the most 
modified in detail of all, from the fish -like scales of a penguin's wings to the glittering jewels of 
the humming-bird, and all the endless array of the tufts, crests, ruff's, and other ornaments of the 
feathered tribes ; even the imperfect bristle-like feathers above mentioned may belong among 
