l^l-ll Chandler: Fcafhrr.i of Circus hurls(*)iius 333 



and slender, some of them reaching a length of 25 millimeters, 

 with a diameter of only 0.02 millimeter. The barbules, aver- 

 aging about thirty-five to thirty-eight per millimeter on each side, 

 are so fine and slender as to be scarcely visible to the naked eye. 

 Though they may reach a length of 4 millimeters, they are scarcely 

 over .005 millimeter in diameter. The individual cells of the 

 barbules are from 0.06 to 0.15 millimeter long, and often bear 

 at their distal ends two tiny prongs, which are rudimentary 

 barbieels. The result of the density and length of the.se barbules 

 is a cottony texture of the feather, and a covering for the body 

 which could hardly be improved upon for the purpose it serves. 

 The innumerable slender barbules are so dense as to produce a 

 waterproof covering, and yet the air spaces between the barbules 

 render it practically a non-conductor of heat, thus affording a 

 verj^ efficient heat insulation. In addition to this, it has an 

 entirely negligible weight, giving therefore a minimum of wear 

 and tear and waste of energy and a maximum of efficiency. 



Powder-Down 

 The powder-down feathers in general strixcture closely re- 

 semble the ordinar.y down feathers of the body, but are at once 

 recognizable by their peculiarly matted appearance, due to the 

 masses of powdery siibstance adhering to the barbs and barbules. 

 The proximal portion of the feather is most heavily laden with 

 the substance, and, in addition, the barbs and barbules are always 

 imperfectly differentiated, remaining partially fused as in a 

 growing feather. There is no evidence of the tips of the barbs 

 or barbules breaking down to produce the powder, as stated by 

 Pycraft (1910), and the fact that the powder is most abundant 

 near the ba.se of the feather would further militate against this 

 view. Nitzsch (1867) suggested that the powder was continually 

 poured forth from the open end of the calamus, but this does not 

 seem probable. The most probable explanation, it seems to me, 

 is that a considerable portion of the feather germ, instead of 

 developing into barbs and barbules, is stimulated by some 

 unknown cause to break down into a fine powder, whicli adlieres 

 to the barbs and barbules, often in regularly shaped oval masses. 



