“Tree-Ducks” of the genus Dendrocygna. 9 
and, externally, it by no means impresses one as being a near ally 
of Anser. 
Through its now recognized nine known species, the genus 
Dendrocygna is represented throughout the lower-temperate and 
tropical regions of both hemispheres, including Australia (D. eyton:i) ; 
and it is quite possible that still others remain to be discovered. 
The comparative osteology of the “Tree-Ducks” of the genus 
Dendrocygna. 
The skull. As in the case of all birds, there is found to be 
an individual variation for the skull of this duck, and this variation 
pertains to both sexes; though, wherever it may occur, it is never 
very great. This statement is made with respect to Dendrocygna 
bicolor and D. autumnalis, for I have not had the skulls of any of 
the latter to compare. In fact, my osteological observations, as set 
forth here, must unfortunately be confined to the skeletons of these 
two species, apart from a few notes which have been kindly sup- 
plied by others. 
It is to be noted that some slight differences are also found 
to occur, when we come to compare the skulls of two individuals 
of the same species. Everything else being equal, the skull in the 
female is usually smaller, too, than in that of the male of the same 
species; while, upon the whole, were we to mix up fifty skulls of 
both sexes and both species of these ducks, and had we nothing to 
go by beyond what these skulls themselves presented, we would 
find it quite diffieult to separate them again, and I very much 
question that it could be done. 
A skull of a male (456) D. bicolor has a transverse zygomatic 
diameter of 28,5 mm, while that of a female of the same species 
measures but 25,0 mm. With respect to their mid-longitudinal 
lengths, however, these two skulls are about equal (from center of 
oceipital condyle to end of superior mandible equals 91,0 mm). 
For the purposes of a general description of the skull in the 
genus Dendrocygna it matters very little which one we select, and 
I therefore, from the several skulls before me, choose the very per- 
feet one of a Dendrocygna autumnalis (454), and all my observations 
refer to it, unless stated to the contrary (see Fig. 12, Pl. 3; Fig. 15, 
Pl. 4; Fig. 54, Pl. 8, and several other figures on the Plates for 
the skull in Dendrocygna). 
