EXTERIOR PARTS OF BIRDS 



127 



ules, just as if the lower edge of the barbules were frayed out, and 

 only diti'er from each other in that barljicels are plain liair-like }iro- 

 cesses, Avhile hamuli are liooked at the end ; they are not found on 

 all feathers, nor on all parts of some feathers. Barbi- 

 cels occur on both anterior and posterior rows of 

 barbules, though rarely on the latter ; booklets are 

 confined to any anterior series of barl>ules, which, as 

 we have seen, overlie the })Osterior rows, forming a 

 diagonal mesh-work. The design of this beautiful 

 structure is evident; the barbules are interlocked, and 

 the whole made a web ; for each booklet of one barbule 

 catches hold of a barbixle from the next barb in front, 

 any barbule thus holding on to as many of the barb- 

 ules of the next barb as it has booklets ; while, to 

 facilitate this interlocking, the barbules have a 

 thickened upper edge of the right size for the hook- 

 lets to grasp. The arrangement is shown in Fig. 22, 

 Avhere a, a, a, a, are four barbs in transverse section, 

 viewed from the cut surfaces, with their anterior, 

 h, h, h, h, and posterior, c, c, c, r, barbules, the former 

 bearing the booklets which catch over the edge of the 

 latter. 



Types of Feathery Structure. — But all feathers 

 do not answer the above description. The after-shaft 

 may be wanting. Hooklets may not be developed, 

 as frequently happens. Barbicels may be few or entirely want- 

 ing. Barbules may be similarly deficient, or so defective as 

 to be only recognised by their position and relations. Even 

 barbs themselves 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- 



Fio. 22.— Four 

 barbs in cross 

 section, a, a, a, a, 

 bearing anterior, 

 6, b, b, h, ami 

 posterior,c, c, c, c, 

 barbules, the 

 former bearing 

 hooklets which 

 catoh over the 

 latter ; magni- 

 f i e (1 ; after 

 Nitzscli. 



Fig. 23 



-A feather from the tail of a kingbird, Tyrannus mroKiiensis, almost entirely 

 pennaceous ; no after-shaft. From nature, by Coues. 



structure has led me to the recognition of three types or plans : 1. 

 The perfectly feathery, ])Jmiious, or pennaceous (Lat. j^hona, a plume, 

 Qv penna, a feather fit for writing with ; Fig. 23), as above described. 

 2. The downy or jylumidaceous {Lat. pluvnila, 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 dilatations in 



