268 University of California Puhlications in Zoology fVoL. 13 



produces the fringe, or edge, usually of a paler color, so frequently- 

 found on the remiges and coverts of passerine birds. 



Usually the structure of barbules, except for variations already 

 mentioned, varies but little on different portions of the same barb or 

 of the same feather. At the base of the barb there is a decided 

 shortening of both kinds of barbules, while at the tip the change is 

 in the nature of a loss of the perfection of structure. Usually, also, 

 the distal barbules of the inner vane have the specialized dorsal 

 cilia better developed on the terminal than on the basal portion 

 of the barb. 



Surveying the entire class of birds, we find that the pennaceous 

 barbules vary considerably in the different orders and suborders, 

 though usually being fairly constant within the lesser groups, except 

 where modified for color production, or other conspicuous effect. 

 Differences occur in size and shape of the bases, position of nuclei, 

 form of pennulum, and nature of all the different types of barbicels. 

 In tinamous alone a most remarkable modification • of the typical 

 vanules occurs in the solid secondary fusion of the pennula of the 

 proximal barbules, except at the extreme tip of the barbule, to form 

 a limiting bar parallel to the barb (pi. 25, figs. 49& and e). Though 

 this surprising modification is absolutely characteristic of all tinamous, 

 not only of the remiges, but of all the other pennaceous contour 

 feathers, I have found no suggestion of it in any other birds, and 

 I have been unable to find any reference to it in the literature. 



2. Rectrices 



With this brief survey of the conditions found in the remiges of 

 birds, we may now turn to the other groups of contour feathers. 

 Next to the remiges, the most highly developed feathers of the body, 

 in birds of strong and graceful flight, are the rectrices. The macro- 

 scopic form of the tail and of its individual feathers varies con- 

 siderably, and the microscopic structure is far more subject to modifi- 

 cations for special functions than it is in the remiges. In normal 

 rectrices, used in flight for steering and balancing, the structure is 

 very similar to that of the inner remiges, and it is interesting to 

 note that in the middle tail feathers both vanes have a type of struc- 

 ture of barbules similar to that characteristic of the outer vane of 

 remiges. In ratite birds there are no specialized rectrices among 

 the Casuariiformes or the Apterygiformes, while in the Struthioni- 

 formes and the Rheiformes the rectrices are large and developed as 



