SHUFELDT. ] OSTEOLOGY OF THE TETRAONIDZ. 681 
The Sternum, in the North American Tertraonide, is developed from five 
points of ossification, and to these it seems to have added, later in life, 
or before the bone becomes one entire piece, an ossific centre at the ex- 
tremity of each of the four lateral xiphoidal prolongations from which 
their subsequently dilated ends are produced. These later are easily to 
be demonstrated in the hemal spine of Centrocercus, in the “bird of the 
year” (Plate VI, Figs. 53 and 56). . 
Fig. 53 represents the young of this last-named Grouse a day or so 
old, at which time all five of the primoidal points of development are 
eminently distinct. The ‘“‘body” of the bone is nearly circular. The 
“Keel,” of which only the anterior part has as yet ossified, dips well 
down between the tender pectorals; the manubrium, now only in carti- 
lage, has at this date no evidence of the foramen that later joins the 
coracoidal grooves. As to the rest, bands of delicate membranous tissue 
bind them loosely together. The sternum in a bird of several months’ 
growth is shown in Fig. 56. Here the bone is rapidly assuming the 
shape it is destined to retain during life. The body and with it the keel 
is extending by generous deposition of bone tissue at its margins, prin- 
cipally at the mid-xiphoidal prolongation. The manubrium, still in car- 
tilage, we find pierced at its base by the foramen just alluded to, and 
a rim of the same material runs about the anterior border of the lophos- 
teon, Fig. 56, 4, while a rapidly diminishing band also connects the ele- 
ments known at this stage as the pleurosteon, ib., 6, and the metosteon, ib., 
5. In cases where severe maceration is resorted to with this bone, in 
still older specimens, in which the sutures are not suspected, these parts 
will still separate about the original points of ultimate union. 
On the reverse side of the bone shown in Fig. 56 we find that even 
at this stage it is deeply perforated by the pneumatic foramen at a 
point immediately over the carinal ridge. 
In the adult the sternum is highly pneumatic, air having access to it 
through such apertures not only at this point but also in the costal bor- 
ders between the sternal ribs, and by a single foramen in the groove, 
posterior to the manubrial process mesiad. 
In Plate VI, Figs. 52, 54, and 55 are all parts of the skeleton of the 
same bird—an old adult Sage Cock, Centrocercus—of which Fig. 54is a 
view from below of the sternum. 
It will be seen that it has a length of 14 centimetres, and other meas- 
urements can be easily obtained from it. We have never seen this bone 
any larger, and, as it is, it represents the maximum size the hemal 
spine attains among North American Grouse. The bone isshown in other 
plates also, and the.. »wners can be ascertained if the reader will kindly 
refer to the description given opposite each plate. 
The unique outline of the sternum of the Gallinw has long been known, 
many authors having both figured and described it, and we will say here 
that in the Tetraonide of our country no marked deviations are to be 
noted from the more common type. 
Anteriorly the manubrium juts out as a quadrate process with 
rounded angles; its inferior margin is continuous with a line that runs 
down between the slightly prominent carinal ridges, to become continu- 
ous below with the anterior carinal margin. 
Above, the general surface of the sternal body extends over it. A 
subcircular foramen, connecting the coracoidal grooves, pierces it at 
its base. The coracoidal furrow, thus becoming one groove, is biconvex, 
being depressed mesiad behind the manubrium, in which depression 
another pneumatic foramen usually occurs. Their upper and lower 
