“Tree-Ducks” of the genus Dendrocygna. 33 
with the atlas as we find it in certain swans for example, as in 
Olor columbianus the articular cup for articulation with the condyle 
of the occiput is not ossified superiorly, the interval having a strong 
ligament spanning it in life. Both of these birds, too, possess, on 
either side of the atlas, a foramen for the passage of the vertebral 
vessels. In Cereopsis, the articulatory cup of the atlas is completed 
in bone, and the vertebral foramina are barely closed in laterally. 
They may even be open in some ducks, as in Polysticta and doubt- 
less in others. As to the neural arch, it is generally broad antero- 
posteriorly and of uniforın width; to this, however, such a form as 
Hymenolaemus malacorhynchus is an exception. 
No haemal spine whatever is present on the atlas in Dendrocygna, 
while that process is quite well developed in Olor, to some extent 
in Branta, and quite conspicuously so in such a duck as Hymeno- 
daemus malacorhynchus. 
Aix possesses an atlas much as I have described it for the 
Tree-ducks, while in some of its details it varies throughout the 
Anseres. 
Passing to. the axis of Dendrocygna autumnalis (454), we find 
its odontoid process well developed, being flat above and convex 
beneath. Below it, the facet for the atlas is transversely elliptical 
‚and moderately concave. 'T'he vertebral canals are formed by thin, 
lateral lJaminae of bone, the postero-inferior angle of either of which 
is inclined to be drawn out into a process, which in Olor columbianus 
is very conspicuously the case. Above, the neural arch is thick 
and broad, increasingly so as we proceed backwards. It supports a 
median, low, thick, neural spine, while the haemal spine below is 
extensive, much compressed from side to side, and slopes away 
posteriorly. At the middle of the base of the haemal spine, it is 
pierced, from side to side, by an irregular foramen. This is also 
present in Olor, but not in Chen or Branta. 
In some of the Sea Ducks, as in Charitonetta, the haemal spine 
‘of the axis vertebra is narrower antero-posteriorly at its base than 
in most Anatinae, thus, to a greater extent, invidualizing this 
‚apophysis. 
The third cervical vertebra in the spine of Dendrocygna 
has the neural process set far back on its arch; while the haemal 
‘one, with much of the form it has in the axis, exhibits considerable 
reduction in size. On either side, the vertebral canal is more 
extended and more tubular. Posteriorly, either transverse process is 
Zool. Jahrb. XXXVIIT. Abt. f. Syst. 3 
