1916] Chandler: Structure of Feathers 267 



thornlike projections, which are directed more or less distad, i. e., 

 toward the tip of the feather (see plates). In the Falconidae they 

 differ from the other dorsal cilia only in being slightly stouter and 

 more thornlike (pi. 23, figs. 36a, 37a, 38a, 39a, 40a) ; in owls they 

 are well developed but not differentiated from the other cilia (pi. 32, 

 fig. 84a) ; while in a few birds, e. g. trogons (pi. 31, fig. 80a), they 

 are absent entirely. On the outer vane, on the other hand, the dorsal 

 cilia of the proximal portion of the pennulum are always absent, 

 at least on the first two or three cells ; very frequently no dorsal 

 cilia whatever are present. Usually, however, following the proximal 

 two or three cells, rudimentary barbicels begin to appear, and these 

 become more and more pronounced distad, the reverse condition to 

 that found on the inner vane. With the exception of these few 

 details, the structure of distal barbules of both inner and outer 

 vanes is usually alike, and both show the same group characteristics. 

 The proximal barbules (fig. E) of the two vanes are nearly always 

 exactly similar in more or less of the basal portion of the barbs, but 

 in the majority of birds they differ in the more distal portion of the 

 barb, sometimes only at the tip, more frequently in from one-third 

 to two-thirds of the terminal portion. Those of the inner vane, and 

 those which are similar to them on the outer vane, have rather long 

 slender bases, considerably longer than the bases of the distals, a 

 series of three to six ventral teeth of differing degrees of develop- 

 ment in different birds, and more or less filamentous pennula with 

 only very rudimentary barbicels if any at all. With a few excep- 

 tions, notably most of the ciconiiform birds, the more distal proximals 

 of the outer vane differ decidedly from the others in the development 

 of a series of ventral barbicels, these being formed as an increased 

 number of ventral teeth, accompanied by a change in form. In 

 many birds these ventral barbicels, homologous to the booklets and 

 ventral cilia of distal barbules, are very large and numerous and 

 highly conspicuous, e. g., in gallinaceous and falconid birds (pi. 23, 

 fig. 38c, and pi. 24, fig. 42e). Perhaps the greatest development is 

 that found in Ceryle alcyon (pi. 31, fig. 79a). In many passerine 

 birds, e. g., in all the Tyrranidae, the outer half of the barbs under- 

 goes a very sudden and conspicuous change from plain to barbicelled 

 proximal barbules (compare plate 33, fig. 92<i with plate 33, fig. 93a), 

 this sudden transition including a very marked reduction in the size 

 of the base, and a concomitant simplification of the distal barbules, 

 the booklets of which become obsolete. This change in structure 



