yo. 2.] SHUFELDT ON NORTH AMERICAN TETRAONID^. 323 



Of the Vertebral Column. — In discuss'ng the development and peculiar- 

 ities of the vertebral column, we will still continue to adhere to Centro- 

 cercus as our model, explicitly stating names of other species when oc- 

 casion requires a departure therefrom. 



In examining the atlas and axis as they occur together in the chick a 

 few days old, we find that the neurapophyses of the first have as yet 

 failed to fairly meet above in the median line; though they may in 

 some instances, as they undoubtedly do, soon touch each other. No os- 

 sific centre exists for an atlantal neural spine, as that process is not found 

 upon this bone in any of the Tetraonidce. Below the arch the interest- 

 ing procedure is progressing in the appropriation of the centrum of this 

 segment by the axis. The inferior extremities of the atlantal neura- 

 pophyses have inserted between them a circular ossicle whose plane is in 

 the horizontal plane and on a level with the floor of the neural canal of 

 the axis. This bonelet eventually becomes the "odontoid process " of the 

 second vertebra. At this stage it is a little less than a millimeter in 

 diameter, and in the adult occupying the same position remains a sub- 

 sessile, and in comparison with the bulk of the bone it is attached to, an 

 insignificant tubercle, though unquestionably fulfilling all the impor- 

 tant functions required of it. In less than six weeks the odontoidal lar- 

 ceny is complete, and no trace remains to lead one to suspect how mat- 

 ters stood at the earlier date. 



Immediately beneath and a little posterior to the i)rimoidal and dis- 

 tiuct centrum of the atlas, there is another, and still larger, free ossific 

 centre, uniform in outline, concave above, surrounding the primitive 

 odontoid apophysis with its long axis lying transversely ; behind, and 

 in contact with it, are two more ver^^ minute and elliptical ones placed 

 side by side. The first of these unite with the atlas and latterly form 

 the bony ring for the occipital condyle to revolve in, and the surface for 

 the odontoid to move upon, and a notched lip of bone that projects from 

 it behind, that subsequently develops — the remaining two, the anterior 

 part of the body of the axis that bears the articulating surface for the 

 first vertebra. In the full grown bird the postzygapophyses of the 

 first vertebra projecting well to the rear look almost directly inward. 

 They meet the prezygax)ophyses of the axis that face in a contrary direc- 

 tion and a little backwards. The articulating facet for the centrum of 

 the axis is subelliptical, convex, of some size, and has in the segmented 

 column the inferior and convex surface of the odontoid i)layiug just 

 above it, the superior and flat surface of the latter being confined by 

 intervening and delicate ligament forming a part of the floor of the 

 neural canal of this bone. In the axis of the adult the anterior part of 

 the bone with the odontoid process, that was separately added, projects 

 conspicuously forwards beyond the neural arch, and in birds of several 

 months of age it can be distinctly discerned where the union was estab- 

 lished between neurapophyses and centrum. 



In the mature vertebra the neural canal is nearlv circular. In the 



