1916] Chandler: Structure of Feathers 303 



is over half the length of the main feather plate, with 30 or more 

 barbs in the breast feathers. 



a) Diomedea exsulans 



(1) Reviex 



Highly specialized as functional liight feather, the barbules in 

 fact possessing more "frills" than in any other feather known. 

 Shaft deeper than wide, especially on more proximal portion of 

 feather, with sharp, inconspicuous groove ; calamus large and in- 

 flated. Vanes firm and elastic ; barbs fairly close together, about 

 18 per centimeter on inner vane, somewhat less on outer, their rami 

 considerably deepened near junction with shaft, and barbules firmly 

 interlocking. Pith of barbs consisting of only a single layer of 

 hollow cells transversely, the ventral ridge not as broad and thin 

 as in loons, but constituting about a fourth of depth of barb. 



Inner vane. — Distal barbules (pi. 17, fig. 10a) with a number 

 of rather striking characteristics. Twist between base and pennu- 

 lum unusually pronounced, making a very sharp curve in dorsal 

 contour of barbules as they lie undisturbed in vanule. Base large 

 and rather long, 0.45 mm. by 0.16 mm., differing from barbs of 

 birds of any other group, as far as known, by the presence of one 

 or two minute forward-projecting prongs on dorsal edge, on cells 

 bearing ventral teeth. Latter very peculiar in form, not simple 

 finger-like or lobate projections, but bifid at tip, and often trifid 

 (pi. 17, fig. 10a). Pennulum longer than base; booklets moderate 

 and well-formed ; ventral cilia long, slender and flexible ; the dorsal 

 cilia with much the same character as in Colymbiformes, i. e., short 

 and triangular basally, then becoming more slender, and flnally de- 

 creasing, those beyond the sixth usually rudimentary or absent en- 

 tirely. Proximal barbules (pi. 17, fig. 10&) large and well formed; 

 base very long and relatively slender; about 0.76 mm. by 0.06 

 mm. with a well-developed flange, and rather small dorsal spines ; 

 ventral teeth differing from those of all other birds in being slender, 

 flexible, and greatly elongated, sometimes 0.15 mm. long, and 

 usually with wavy appearance, as shown in figure. Pennulum ap- 

 proximately equal to base in length, broad at proximal end (about 

 0.01 mm.), tapering gradually to tip. Distal barbules set very 

 close together, about 30 per millimeter, proximals, being set at a 

 much more acute angle, much less numerous, about 17 per milli- 

 meter. 



Outer vane. — Distal barbules (pi. 17, fig. 10c) differing from 

 those of inner vane in same manner as in loons, i. e., base slightly 

 more elongate, booklet region of pennulum broader, booklets more 

 numerous, ventral cilia longer and more filamentous, and dorsal 

 cilia undeveloped except terminally, but specific characteristics, 

 such as dorsal prongs and jagged ventral teeth, unchanged. Prox- 

 imal barbules (pi. 17, fig. lOd) with shorter and more tapering 

 base, and extremely long, heavy pennulum with a complete series 

 of ventral barbicels, the more proximal of which long and wavy. 



