326 BULLETIN UNITED STATES GEOLOGICAL SURVEY. Wol.Xl. 



hypapopbysis is still present anteriorly witli a tubercle developing on 

 either side of it, with the parietes of the vertebral canal very much 

 slenderer. In examining this segment in the younger bird we ascertain 

 that the original ossicle is now a descending pleurapophysis meeting 

 the parapophysis, a delicate and indeijendent process, which, in the fif- 

 teenth and last cervical vertebra, constitutes a free rib, while the hypapo- 

 physis consists of a mid-process and a smaller nodule on either side. 

 This beautiful metamorphosis can be thoroughly studied and easily com- 

 prehended in the cervical jjortion of the vertebral column in our Gatliar- 

 tes aura. 



So that, as a partial recapitulation of the first fifteen segments, we 

 find that they make up the "cervical portion" of the column. Their cen- 

 tra are universally subcompressed at their middles, they develop in the 

 young bird jDarapophysial projections that eventually produce free ribs 

 by the aid of the descending pleurapophyses, and their iuterarticula- 

 tions, as far as their bodies are concerned, bear out the general ornithic 

 law of being apparently procoelous on vertical section and opisthocoe- 

 lous on horizontal section. 



Backwards from the fifteenth the vertebral segments or the links of 

 the chain take on a metamorphosis that is characteristic of the Tetraon- 

 id(G. It consists in, in all the adults of the genera, a consolidation of 

 the ensuing four vertebra. The confluent bone thus formed constitutes 

 the major part of the dorsal division of the spinal column and invariably 

 supports free pleurapophyses (Plate YI, Fig. 55, Centrocerc-us, ad. ^ ). 



In Centrocercus these four vertebraj can easily be distinguished from 

 each other until the bird is over a year old, but very soon after this all 

 sutural traces are entirely obliterated and we have the segment as rej)- 

 resented in the plate. 



The neural spines become one long parallelogrammic plate, occasion- 

 ally exhibiting a foramen or so at the site of the original interspiiious 

 spaces. 



Its crest is rounded, but has no independent rim. Muscular fascia at- 

 tached to it i)osteriorly often ossifies, leaving in the prej^ared skeleton 

 flattened spiculse, on either side, directed backwards. The anterior 

 aspect of this bone has all the necessary elements to meet the last free 

 vertebra beyond it. The first pair of diapophyses are the shortest, the 

 last pair the longest and most raised ; these processes are more or less 

 bound together by metapophysial ofi'shoots of variously defined serrate 

 margins, that allow interdiaj)ophysial vacuities to exist. Below, and 

 just anterior to the bases of the diapophyses, are the four subelliptical 

 and concave facets of the cai^itula of the dorsal pleurapophyses. From 

 their ujjx^er and iJosterior i^oints sharp crests run beneath the transverse 

 processes to meet the out-turned and cordate facets at their extremities 

 for the tubercula of the ribs. At regular intervals, and nearly in a 

 right line among the diapophyses, are the elliptical orifices for the 

 transmission of the dorsal nerves. 



