“Tree-Ducks” of the genus Dendrocygna. 17 
which 1 offer in Fig. 12 of Plate 3 of this paper. Here it will be 
appreciated how very different this part of the skull in the Tree- 
Ducks is, as compared with the superior mandible of other Anatidae 
shown on the same Plate, as Cereopsis, Hymenolaemus, and Chloephaga 
poliocephala. In fact, without making any extended comparisons, it 
is clear, upon comparing the superior mandible of Dendrocygna 
autumnalis as seen, not only upon this lateral but on any view, that 
is it, morphologically, more like the superior mandible of such a 
duck as Anas platyrhynchos (Fig. 38, Pl. 6) than like this part of 
the skull in any of the Swans or Geese, or even some other Ana- 
tinae, as for example Marila marila or Oidemia velvetina. Such a 
form as Spatula clypeata need not be mentioned in this connection, 
as the form of its superior mandible is unique, absolutely, among 
true ducks. This fact I have fully demonstrated in previous publi- 
cations, particularly in my “Osteology of Birds”, published by the 
State Museum of Albany, and “Observations upon the Osteology of 
the North American Anseres” (in: Proc. U. S. nation. Mus., 1888). 
The free dentary margin of the side of the superior mandible 
is more or less concave between distal extremity and the naso- 
maxillary termination posteriorly in Chen, Anser, and other geese, 
while this border is convex in Dendrocygna, Anas, and many other 
ducks. 
All Anatidae, in so far as I have examined their skulls, 
possess large, open, subelliptical external narial apertures, with no 
osseous nasal septum standing between them mesially. These aper- 
tures are differently located in the different genera of the Ana- 
tidae. Tree-Ducks, Mallards, Black ducks (Anas rubripes) etc. have 
the narial opening on either side high up in the basal half of the 
mandible. In Anser, where they are very large, they encroach on 
the middle third; while in such as form as Chloephaga hybrida, they 
oceupy the middle third of the side of the mandible (Fig. 35, Pl. 6). 
Their comparative size and position in other anserine fowls may 
easily be seen by examining the various skulls of the Anatidae 
on the Plates illustrating the present paper. 
Throughout this entire assemblage of birds, on the lateral view 
of the eranium posteriorly, the external aural aperture is a con- 
spieuous feature. Morphologically, this opening and its surrounding 
region varies with the species; but the differences presented shed 
but little light upon the question of probable affinities. 
Dendrocygna has the aperture rather large in proportion to the 
Zool. Jahrb. XXXVIII. Abt. f. Syst. 2 
