1916] Chandler: Structure of Feathers 271 
similar to that of the proximal vanules of the downy barbs of the 
feather proper, in case there is any special modification in the distal 
vanules, as in gallinaceous birds. 
c) Pennaceous Barbules—Concerning the pennaceous portion of 
contour feathers of the trunk, all degrees of development of structure 
can be found. In ratite birds, as is well known, there is never any 
pennaceous structure developed, although the bases of the barbules 
in Rhea (pl. 13, fig. 1a) are exceptionally well developed for down, 
and seem to indicate a transition to or from a pennaceous type of 
barbule. In typical trunk feathers there is no differentiation of 
inner and outer vanes, and usually the structure is a mere simplifica- 
tion of that found in the more highly specialized remiges. In the 
contour feathers of the trunk, as would be expected, the distal bar- 
bules are of the type of the outer vane of the remiges, and the proxi- 
mal barbules of the type of the inner vane of the remiges, often in 
very degenerate form, these types being the ones showing the lesser 
degree of specialization. In the trunk feathers the conspicuous basal 
dorsal cilia are seldom developed on distal barbules, and ventral cilia 
seldom occur on the proximal barbules (pl. 20, figs. 20e and f). Ina 
great many birds these structures in trunk feathers are very much 
simpler than they are in the remiges, all of the barbicels being very 
much reduced or even absent. In distal barbules the cilia often 
disappear entirely, the hooklets are reduced to one or two very weak 
ones, and the ventral teeth are represented only by a very small, 
inconspicuous projection (pl. 33, fig. 92e); the proximal barbules 
frequently lose the sharp differentiation between base and pennulum, 
becoming evenly tapering all the way to the tip (pl. 33, fig. 92f). 
Such modifications are always farther advanced on the breast and 
belly feathers than on those of the back, the back feathers often 
being intermediate between the remiges and coverts on the one hand, 
and the breast and belly feathers on the other. 
In some birds the pennaceous barbules of trunk feathers have 
special modifications of their own, and, as might be expected, these 
are usually more conspicuous and better developed on breast than 
on back feathers. The most peculiar structural modification charaec- 
teristic of trunk feathers only, and the only one which needs special 
mention here, is the development of curved dorsal barbicels on the 
base of both distal and proximal barbules (pl. 17, fig. 10e, 12a, b). 
Since these barbicels are not homologous with any other types of 
