SHUFELDT.] OSTEOLOGY OF THE TETRAONIDA. 683 
be recognizad by its superior xiphoidal processes, Bonasa by the nar- 
row body, Centrocercus by its size in the larger specimens, and so on. 
We will still continue to consider such of the vertebral column as is 
confluent in the old bird, or rather such vertebra as become confluent 
and are more or less embraced by the ossa innominata, as the sacrum, 
and composed of sacral vertebree, attempting to make no such divisions 
as Professor Huxley did, in his Anatomy of Vertebrated Animals, of 
this compound bone, though we must believe that this author is emi- 
nently correct in the view that he’takes of this bone. 
There are sixteen of these segments that are to be so reckoned in 
Centrocercus, but itis only in the “bird of the year” that they can be - 
counted with anything like accuracy, and even then great care must be 
exercised, and various pelves examined and compared with the younger 
birds at different stages and ages. 
The first sacral vertebra possesses free pleurapophyses, whose hema- 
pophyses do not reach the costal borders of the sternum, but articulate 
in a manner to be described furtheron. Regarding the pelvis from be- 
low in Centrocercus, we note that the anterior four sacral vertebre have 
their combined par- and diapophysial processes thrown out as braces 
against the expanded anterior iliac wings. After this the ilia change 
their form to accommodate themselves to the basin of the pelvis, which 
they assist in inclosing, and with this change the succeeding vertebra 
have their diapophyses much elevated to meet the internal iliac margins. 
This section consumes four additional vertebra, the centra of which 
go to make up the latter moiety of the cavity for the “ventricular dila- 
tation” of the myelon, and they show the double foramina on either 
side, one above another, for the separate exit of the motor and sensory 
roots of the sacral plexus. 
A double row, 7. é., one on either side of subcircular vacuities, exists 
here also among the transverse processes (Plate XIII, Fig. 90, Canace 
canadensis, 8). It is through this portion of the sacrum that we observe 
in the chick the greatest amount of tardiness in sealing up of the neural 
tube above by the superior union of the engaged neurapophyses. 
The remaining eight vertebre become much compressed with ex- 
panded processes that rarely allow apertures to remain among them, 
forming an excellent mid-section to the broad and capacious pelvic 
cavity, with nearly all signs of its original formation obliterated on the 
outer and superior aspect. 
The neural canal is distinctly circular as it enters the sacrum an- 
teriorly, becoming only slightly flattened as it nears the coccyx. 
Above we find the neural spine confluent with the ilia anteriorly along 
its summit, and some additional bone deposited posteriorly in the way 
of their lateral plates, to bridge over the ample ‘“ilio-neural” canals. 
Opposite the “ gluteal ridges” the bones are yet firmly knit, but for the 
remaining part of the sacro-iliac suture the interested bones can be 
said only to snugly meet each other. Cupidonia alone has quite an in- 
terspace present (Plate XII, Figs. 83 and 84). 
The sacral wedge is quite ‘thoroughly permeated by air, which enters 
through foramina in the vertebre in localities similarly situated to 
those described in speaking of the anterior part of the column. 
In Centrocercus sometimes the first sacral vertebra bears a well-de- 
veloped hypapophysis, and there may even be some evidence of this 
process on the second segment. The expanded extremities of these 
median processes are connected along their inferior margins by a deli- 
cate ligament of a firmer consistence than that membrane, which fills 
in the vacuities between the processes to which it is a limiting border. 
