148 Zoologica: N. Y. Zoological Society. [I; 5 



the parrot Prioniturus. The barbs are scanty, but probably are 

 all present when the shaft first emerges from the blood sheath. 

 Each barb breaks off at the proximal end of its line of green pig- 

 ment. The most noticeable character is the great degeneration 

 in length of these barbs. At the base of the racket the barbs 

 measure one and a quarter inches in length, while barely a half 

 inch farther up the shaft, the barbs are reduced to one-quarter 

 of an inch in length. The dropping off of the barbs is almost 

 contemporaneous with their breaking from the sheath, the shaft 

 being bare long before it has reached its full growth. 



Judging from the condition of the rectrices in these two 

 moulting individuals of Eumomota superciliaris, the evolution of 

 tail denudation is more advanced in this species than in any of 

 the others I have examined. 



A curious asymmetrical condition was observed in a speci- 

 men of Momotus subrufescens in the collection of the American 

 Museum. The character is apparently individual, as it is not 

 present in other specimens. On holding one of the rectrices, 

 which is intact, up to the light, the longitudinal series of holes 

 which indicate degeneration is observable only along the inner 

 life of the shaft. In the other rectrice the inevitable result of 

 this condition is demonstrated in the asymmetrical denudation, 

 the outer web being entirely absent while the inner web is per- 

 fect, and shows no hint of losing even a single barb. This is 

 additional proof that the denudation is wholly dependent on the 

 congenital weakening of the barbs. 



Part VI. 



My conclusions as to the problem of rectrice denudation in 

 the Motmot are as follows: For some reason, totally unknown 

 to us at present, a certain definite portion of the central rectrices 

 of these birds exhibits congenitally a decided degeneration of the 

 barbs and barbules. 



These weakened barbs are usually strong enough to resist 

 the ordinary wear and tear of the feather during its growth. 

 The natural oil or moisture of the feather may not totally evapo- 

 rate from this terminal portion until growth at the base ceases, 

 although there can be no direct communication between the two 

 points. It is a minor consideration whether the final increase of 

 weakness is due to thorough drying and consequent brittleness 

 or to some other physiological cause. 



The Motmot, in the course of the preening to which it sub- 

 jects all of its rectrices, breaks off the barbs in the area most 



