OSTEOLOGY OF BIRDS 205 



cipally 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 lophosteon, while a rapidly diminishing band also connects 

 the elements known at this stage as the pleurosteon, and the metos- 

 teon. 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 we find that even at this stage 

 it is deeply perforated by the pneumatic foramen at a point imme- 

 diately 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 borders between the sternal ribs, and by a single foramen 

 in the groove, posterior to the manubrial process, mesiad. 



In old males of this species, the sternum may attain a total length 

 of 14 centimeters ; I have never seen it larger than that, and it is 

 not exceeded in size by any other grouse in this country. 



Passing to the bones of the shoulder girdle in Centrocercus, we 

 are struck with its comparatively short scapulae, and its peculiar 

 os furcula, wherein the clavicular limbs closely approach each 

 other, being at the same time nearly parallel ; while produced 

 directly downward, below, is the long hypocleidium, which is ex- 

 panded, as usual, posteriorly. Of these hones, only the large cora- 

 coids seem to enjoy pneumaticity. 



The bones of the shoulder girdle are all well advanced in ossifica- 

 tion in the young chick, but do not exhibit their salient characters 

 until a bird is pretty well along in age ; this applies more particularly 

 to the muscular lines on the shafts, the base of the coracoids, and the 

 clubbed extremities of the scapulae. 



The os humero-scapulare does not develop in bone in any of the 

 Gallinae in our avifauna. A firm piece of inelastic cartilage is 

 found to exist in its place, which serves to increase the articular sur- 

 face of the glenoid cavity of the shoulder joint. Centrocercus pos- 

 sesses a humerus which in the matter of size is in due keeping with 

 the rest of the bird's skeleton. It presents all the ordinary ornithic 

 characters of this bone of the pectoral limb, as they are to be found 

 among the Carinatae generally. Its usual sigmoidal curves are fairly 

 well pronounced, and its shaft is somewhat compressed from side to 



