314 University of California Publications in Zoology  \Vou.18 
should be compared with plate 17, figures 12@ and 12b. Pelecanus 
californicus has the same type of structure. 
(5) Phacthontidae 
The Phaéthontidae or tropic birds, as far as feather structure 
is concerned, seem to show a perfect transition from the Procel- 
lariiformes on the one hand to the Laro-limicolae on the other, 
though apparently more closely related to the latter. As shown by 
plate 19, figure 19a (Phaéthon rubricauda), the base of the distal 
barbules of the inner vane is relatively large and broad, and the 
ventral teeth long and slender, with a very slight tendency to 
bifureate, not always displayed, however. The hooklets are only 
four in number, and relatively short and small, the ventral cilia 
are much reduced, and the dorsal ones likewise, except the basal 
two, which are strongly reminiscent of Laro-limicolae. The prox- 
imal barbules (pl. 19, fig. 19b) differ from those of other Stegano- 
podes, but agree with the Laro-limicolae in their relatively small 
size, and in having small inconspicuous ventral teeth. There is 
another significant difference in the relative number of distal and 
proximal barbules. In all other Steganopodes there are nearly 
twice as many distals as proximals, while in Phaéthon there are 
22 or 23 proximals to 30 distals, this genus thus resembling both 
the Procellariiformes and the Laro-limicolae. The barbules of the 
outer vane, the forms of which are shown in plate 19, figures 19c 
and 19d, are characterized primarily by their slender form, the 
wide separation of the hooklets of the distal barbules, which are 
all of nearly equal length, and the weak ventral cilia of the prox- 
imal barbules. The rami of the outer vane have the ventral edge 
serrate and broken into villi, a condition which reaches the height 
of its development in the Anseres. 
The barbules in back feathers of Phaéthon rubricauda closely 
resemble those of the inner vane of the remiges, except in their 
smaller size. 
The breast feathers, as in Pelecanus among Steganopodes, and 
like the Procellariiformes and Laro-limicolae, develop flexules, but 
not aS numerous or as well-formed ones as in Pelecanus. Plate 
19, figure 19e, shows a proximal barbule from the terminal por- 
tion of a barb from a breast feather, while figure 19f of the same 
plate shows a distal barbule from a portion not quite so near the 
tip. Its base is much reduced, and it has weak barbicels; a little 
more distally it assumes a form similar to that represented in plate 
