346 University of California Publications in Zoology [Vol. 11 



proximals at the tip. These under coverts have five or six downy 

 barbs springing from the umbilicus, which may be interpreted 

 as a rudimentary aftershaft. 



Middle and Lesser Upper Coverts 

 All the upper wing coverts except the greater ones lie nor- 

 mally with a reversed overlap, and have well-developed downy 

 aftershafts, which are about half as long as the feather proper 

 in the middle coverts, and still longer in the lesser coverts. 

 The downy barbs of these aftershafts are over three millimeters 

 long, and are silky in appearance. There are over forty such 

 barbules per millimeter on each side. 



The main shafts of all the coverts except those of the greater 

 series are wider than deep, the dimensions shortly above the 

 umbilicus of a middle covert being about 0.30 by 0.54 millimeter. 

 The vanes are perfectly similar as regards their finer structure. 

 The barbs near the base of the feather, about forty per milli- 

 meter, are downy distally, the line of demarcation between the 

 downy and differentiated barbules proceeding in an even curve 

 from the umbilicus until it reaches the tips of the barbs at about 

 one-third the length of the feather. These basal barbules show 

 the transition from one type of barbule to another in a very 

 instructive manner (pi. 20, fig. 21). The distal barbules are 

 transformed by a lengthening of the tip, a shortening and nar- 

 rowing of the base, and a reduction, and finally a loss, of the 

 barbicels and booklets. The proximal barbules change in nearly 

 the same way, the tip elongating and the base becoming nar- 

 rower and more tapering, until finally it is reduced to the 

 narrow base of the downy barbule. The fully differentiated 

 barbules of these feathers are not as highly developed as those 

 previously studied. The proximal barbules, about twenty-three 

 per millimeter, are very narrow, without a sharply defined bend, 

 and with a very slender, threadlike tip, having no rigidity. 

 The distal barbules, twenty-six per millimeter, have only two or 

 three weak booklets, and have very short and rudimentary barbi- 

 cels on the side opposite the booklets; the nuclei are quite 

 indistinct. The lesser coverts closely resemble the middle ones 

 in general structure, but become progressively smaller in sue- 



