72 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4TH SER. 
more or less white. A male (No. 10003, Monterey Bay, Calif., 
Aug. 2, 1907) typically exemplifies “D. gibbosa’. The ab- 
domen and upper and lower tail-coverts are white, partially 
clouded on the abdomen by the gray tips of some of the 
feathers. The white of the head is much extended. The upper 
mandible is “swollen and raised” and “rises high up on the 
forehead.” In another male (No. 10004, Monterey Bay, 
Calif., July 18, 1907) the bill is even more swollen, but the 
white of the abdomen and upper tail-coverts is obscured by the 
dark tips of the feathers. Like variations in the form of the 
bill occur in D. irrorata. 
In some of the specimens new white feathers with dark tips 
have made their appearance beneath the surface in the pos- 
terior white areas. At first glance these sprouting feathers 
seem to be new dark ones appearing amongst the old white 
feathers. A similar stage in the moult evidently misled Mr. 
Salvin into the belief that dark birds are the adults.* In re- 
viewing the British Museum material, Dr. Godman states in 
general terms that he fails to find support for Mr. Salvin’s 
conclusion.” 
While it is believed that the white coloration is due to 
advancing maturity and not to dichromatism, the specimens 
in the Academy’s collection do not fully complete the chain of 
evidence. Judging from those in hand, the white of the abdo- 
men and tail-coverts probably appears first at the base of the 
feathers, then extends with successive moults, finally prevail- 
ing over the dark color. Both on the head and the posterior 
parts the wear of the feathers assists in enlarging the white 
areas. ‘The basal portions of the tail also probably whiten in 
the progress towards maturity. A Laysan Island female (No. 
5615 Stanford Univ.), passing from the natal down into the 
definitive feathers, exhibits no trace of white on the abdomen 
and tail-coverts. 
Although seven months of the year are represented in the 
dates of capture, the Academy’s series is not strong except in 
July and November specimens ; of the former there are eighteen 
and of the latter fifty-five. Three June specimens and the July 
ones are undergoing a moult that is apparently postnuptial. 
> Monograph, p. 446 
2 Mon. Petrels, p. 334, 
