No. 2.] SHUFELDT ON NORTH AMERICAN TETEAONIDiE. 329 



terior borders of the bajraal ribs of the last dorsal pair usually about 

 tbe juuction of tbeir middle and lower tbirds of the bodies of these 

 bones. 



The distal or upper extremities of these, the last pair of hoema- 

 pophyses, are sometimes very much expanded, and in a specimen of 

 Ccntrocercus, 9 , a small bit of bone has been superadded, simulating an 

 additional hcemal spine on either side, as if anticipating the descent 

 of another pair of sacral ribs to protect this otherwise feebly guarded 

 region of its owner's anatomy. 



The short pair of xiphoidal processes overlap the ultimate sternal ribs 

 of all the Grouse and Quails, on either side, as do the ilia the sacral pleu- 

 rapophyses above. 



The Sternum, in the North American Tetraonidcc, is developed from j^re 

 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 hcTsmal spine of Centrocercus, in the "bird of the 

 year" (Plate VI, Figs. 53 and 5G). 



Fig. 5.J 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 looselj' 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, 

 principally at the mid-xiphoidal prolongation. The manubrium, still in 

 cartilage, 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 jioint 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. 



Thus it is that this bone is generated, and as an entirety we are weU 



