272 University of California Puhlications in Zoology [Vor.. 13 



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- 

 fornies, and Laro-limicolae (pis. 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 (pi. 16, fig. 8e) 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 booklets are lost, since booklet 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 booklets (pi. 26, figs. 52d, e, and /). In proximal bar- 

 bules the flexules develop first at the distal end of the base as a 

 direct continuation of the pennular series (pi. 16, fig, 52gr, 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 Qoiira and the upper tail coverts of peacocks, by an uneven 

 decomposition, resulting in the production of ocelli, rackets, etc. 



Although in such decomposed vanes as are found in the 

 "aigrettes" of herons, in the crest feathers of Goura, 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 



