272 Unwersity of California Publications in Zoology (Vor. 18 
barbicels, and are always of the same curved form, they have been 
given a special name, flexules. 
Though totally absent in the majority of birds they are very 
characteristic of several groups, namely, Procellariiformes, Grui- 
formes, and Laro-limicolae (pls. 17, 26 and 28). They are usually 
not present on some of the basal barbules, but are generally charac- 
teristic of a considerable portion of the distal vanule, and usually 
a little less of the proximal vanule. In distal barbules they first 
develop at the proximal end of the base (pl. 16, fig. 8¢) and progress 
toward the pennulum, ultimately forming a continuous series with a 
similar series of pennular dorsal barbicels. This, however, does not 
happen until the hooklets are lost, since hooklet cells never possess 
dorsal barbicels of any sort. The result of this is often a conspicuous 
break in the dorsal series of barbicels on the barbules which still 
retain the hooklets (pl. 26, figs. 52d, e, and f). In proximal bar- 
bules the flexules develop first at the distal end of the base as a 
direct continuation of the pennular series (pl. 16, fig. 52g, h). 
4. Ornamental Plumes 
Frequently some of the contour feathers of the trunk are espe- 
cially modified as ornamental plumes, the variety of form displayed 
by them being very great. There is hardly any group of contour 
feathers which may not at one time or another, in different groups, 
become modified as ornamental plumes. Among such feathers may 
be mentioned the diverse kinds of crests developed in many birds, 
the ‘‘aigrettes’’ of various species of herons, the ruffs and tail plumes 
of pheasants, the gorgeous upper tail coverts of peacocks and tro- 
gons, and the very great number of feather modifications in the 
various species of birds of paradise. Nearly all of these modified 
plumes are produced either by a mere elongation of the feathers 
concerned, by an even decomposition of the vanes, or, as in the 
crest of Gowra and the upper tail coverts of peacocks, by an uneven 
decomposition, resulting in the production of ocelli, rackets, ete. 
Although in such decomposed vanes as are found in the 
‘‘aiorettes’’ of herons, in the crest feathers of Gowra, or in the com- 
mercial ‘‘paradise-plumes’’ (chiefly the under wing coverts of Para- 
disea apoda), the barbs are widely separated from each other on 
the shaft, and appear macroscopically to be devoid of barbules, closer 
examination shows that a more or less complete series of degenerate 
barbules are present, closely appressed to the shaft. In very few 
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