340 BULLETIN UNITED STATES GEOLOGICAL SUEVEY. lVo!!.VI. 



Odontoplwrince, of a trihedral phalanx for the first metacarpal, constitut- 

 ing the index ; two more for the medius metacarpal, the superior joint 

 or 5)halanx having the usual expanded blade, with the long free and 

 pointed finger-bone below; and finally the smallest phalanx of all, freely 

 attached to the distal end of the third metacarpal or annularis. 



These bones are non-pneumatic, as are all the bones beyond the 

 brachium in the Tetraonidce. 



Of the Pelvis and the Pelvic limb. — After what we have said and seen 

 in regard to the dilatory manner in which originally primitive elements, 

 in these birds anchylose, and only after the lapse of weeks condescend to 

 amalgamate and form the confluent bones and cavities that occur in the 

 major division of the Class, we must not be surprised to find the same 

 routine and a like tardiness exhibited in the pelvic haemal arch, or the 

 pelvis, and its appendage the lower extremity. 



A glance at the figures illustrating the condition of the bones in ques- 

 tion, of birds from one to two months old, will convince us at once that 

 the rule still obtains ; in them we find the sutures among the ossa in- 

 nominata still ununited, and the three bones of either side of the pelvis 

 independent, and easily detached from each other about the cotyloid 

 ring. 



In the young chick of Centroeercus (Plate VIII, Fig. 62), the ilium 

 is a scale-hke bone that rests against the sacral vertebrae. The pre- 

 acetabular portion is excessively thin and delicate, and at this period 

 constitutes the longest and widest part of the bone ; its border mesiad 

 bears no vertebral impressions, but is sharp and brittle; the outer margin 

 is slightly rounded ; this condition increasing as we near the diminutive 

 acetabulum, where it is the stoutest. The distal margin, imperceptibly 

 continuous with the inner presents a convex curve anteriorly ; the in- 

 cluded surface viewed from above has a general concave aspect, espe- 

 cially near its central portion. As we proceed backwards, however, it 

 gradually becomes convex, to rise over the region opposite the cotyloid 

 ring to form the general convex surface of the post-acetabular portion. 



The distal margin of the bone is nearly square across, and does not 

 extend as far back as the ischium, the two being bound together at this 

 age by cartilage, which has been removed in the plate. The outer and 

 anterior margin of this division of the bone is rounded and fashioned 

 to the cotyle, the anterior half of which it eventually forms. From this 

 point it arches backwards over the future ischiatic foramen. The inner 

 margin of the post-acetabular portion is rather more rounded than its 

 anterior extension, and directly opposite the "ring" presents for exami- 

 nation the depressions of the transverse processes of the ninth and 

 tenth sacral vertebrae in elong.ated and concave facettes. The ninth 

 vertebra met the ilia first; so we may consider the ilia as the pleura- 

 pophyses of the ninth sacral vertebra, and the ischia and pubic ele- 

 ments as the haemapophyses and divided haemal spine, respectively, as 

 associate parts of the same arch, which arch has been exaggerated to 



