326 BULLETIN UNITED STATES GEOLOGICAL SURVEY. \Vol.Yl. 



liypapopLysis is still present anteriorly witli a tubercle developing on 

 eitlier 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 i^leurapophysis meeting 

 the parapophysis, a delicate and independent 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 i)ortiou of the vertebral column in our Cathar- 

 tes cmra. 



So that, as a i^artial recapitulation of the first fifteen segments, we 

 find tluit they make up the "cervical i)ortion" of the column. Their cen- 

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

 young bird i^arapophysial projections that eventually produce free ribs 

 by the aid of the descendiug pleurapophyses, and their interarticula- 

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

 law of being apparently procoelons 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- 

 idw. It consists in, in all the adults of the genera, a consolidation of 

 the ensuing four vertebrie. The confluent bone thus formed constitutes 

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

 supports free pleurapophyses (Plate VI, Fig. 55, Oeutrocercus, ad. cf ). 



In Centrocercus these four vertebra 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 rep- 

 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 interspiuous 

 spaces. 



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

 tached to it x>osteriorly often ossifies, leaviug in the prex^ared skeleton 

 flattened spiculoe, 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 lougest and most raised ; these processes are more or less 

 bound together by metapophysial ofl'shoots of variously defined serrate 

 margins, that allow interdiapophysial vacuities to exist. Below, and 

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

 and concave facets of the capitula of the dorsal pleurai^ophyses. From 

 their upper and posterior points 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. 



