1916] Chandler: Structure of Feathers 329 



but the barbicels of the proximals of the outer vane are stouter and 

 more irregular. 



Olor columbia7ius has the elongation of the ventral teeth of the 

 distals less extreme, as in Anas, and has the booklets even more 

 slender than in typical ducks and geese. 



Speculum feathers occur in a very large number of genera, espe- 

 cially of ducks, sometimes being white, but frequently some iridescent 

 color, as blue, green, or violet, the structure in these cases being of 

 the type described above for Anas platyrhynchos. A distal barbule 

 from the brilliant green portion of a speculum feather of Nettion 

 carolinense is shown in plate 21, figure 29&. The deep velvety black 

 scapular feathers of Mareca, which are tinged with metallic green, 

 have distal barbules in which the base as well as the pennulum takes 

 part in the color effect (pi. 21, fig. 30a-). 



c) Down 



The down barbules of all typical ducks are short, seldom over 1 

 mm. long, and usually considerably less. They are simple and thread- 

 like for the greater part of their length, but on the basal half of the 

 barbs there are developed at the tip of the barbules 3 or 4, sometimes 

 5, very conspicuous expanded nodes followed by a slender tip (pi. 35, 

 fig. 104). On the outer portion of the barbs these enlarged nodes are 

 reduced and there are a few terminal pairs of prongs taking their 

 place. The number of terminal nodes differs to some extent in dif- 

 ferent species, e. g., Anas has 2 or 3, Mareca 3 to 5, and Mergus 2 

 to 4. In Branta there are 4 to 6, which are not so large and are 

 farther separated. In Olor they are still more separated, less con- 

 spicuous, and the transitional nodes on either side are better devel- 

 oped (pi. 35, fig. 103). 



d) Relationships 



The feathers of the Anseres show high specialization in a num- 

 ber of points of their microscopic morphology, and are unques- 

 tionably to be regarded as the end of one line of evolution. The 

 typical ducks show the specialized characters in their highest develop- 

 ment, the geese, as represented by Branta and Chen, being lower in 

 the scale, and the swans, as represented by Olor, still lower and form- 

 ing a more or less natural bridge over the gap between the more 

 typical Anseres on the one hand, and the Phoenicopteri on the other. 



