MORPHOLOGICAL REVISION OF THE SUBORDER. 109 



antero-posteriorly. The upper end has a large area for ligamentous attachment and the 

 front edge which extends out over the atlas has rugosities for the same purpose. On 

 the upper edge of the anterior articular face of the axis are a pair of concave facets lying 

 at the base of the anterior zygapophyses. On the upper edge of the posterior articular 

 face is a second pair of flattened facets which correspond to the anterior ones in posi- 

 tion. Similar but less well-developed facets appear on the succeeding vertebrae 

 throughout the presacral portion of the column. These facets seem to be different 

 from any described heretofore and, as they have evidently a somewhat similar function 

 to the zygosphene and zygantrum, they have been called by analogy centrosphene and 

 centantrum. These are evidently imperfect in function and development and are 

 variably developed in the diflferent genera and species of the suborder Pelycosauria. 



The third cervical vertebra (description of column to first caudal from No. 

 4008 Am. Mus.) : The centrum is shorter than that of the axis and has the bottom 

 line nearly straight along the edge of a well-developed keel. The posterior face is 

 lower than the anterior, preserving the drop in the neck. The transverse process is 

 short and inclined to the rear; the outer end is free from the centrum and has a well- 

 developed facet for the tuberculum. The zygapophyses are nearly horizontal and the 

 neural spine is relatively short, but higher than that of the axis. The intercentrum 

 has facets for the capitulum of the rib on slight prominences. 



In ih^ fourth cervical vertebra the sides of the centrum begin to show the sharp 

 compression below the intercentrum which forms the narrow high keel in the dorsals. 



In they?/?// cervical vertebra the compression of the sides of the centrum is very 

 marked and the keel very thin and high. The tranverse processes stand out nearly 

 straight and are short. 



In the sixth cervical vertebra the centrum is greatly shortened on the bottom line 

 by the bending back of the anterior face to accommodate the intercentrum. The 

 sides are sharply contracted just below the intercentrum and the keel is nearly half as 

 high as the whole centrum. The centrum is higher than long on the bottom line. 

 (Length of the bottom line 23 mm. ; height of the centrum just posterior to the 

 neural arch 35 mm.) 



The seventh to fourteenth vertebrae (anterior dorsals) are of the same fonn as the 

 sixth, but show a gradual decrease in the bending back of the anterior face below, 

 showing that the intercentrum is growing smaller. The keel on the bottom line 

 remains shorter than the height of the centrum and is very thin ; it extends up to just 

 below the notochordal canal. The anterior articular face has a very wide inferior pro- 

 cess for the intercentrum. The transverse process stands nearly straight out from the 

 upper edge of the centrum. It shows here the origin characteristic for all the geniis ; 

 a ridge rises from the base of the anterior zygapophysis and runs posteriorly ; a ridge 

 rises from the base of the posterior zygapophysis and runs anteriorly. These unite 

 and form the upper portion of the transverse process, the ridge continuing as the 

 anterior and posterior edges. Below a ridge rises from the side of the centrum and 

 fonns the bottom of the process. The process is thus primarily triangular in section, 

 but the position of the lower edge is changeable. At first it extends from the trans- 

 verse process backward, in some forms reaching to the posterior edge of the centrum ; 

 in the dorsals it rises directly beneath the process and extends straight out; in the 

 posterior dorsals it swings forward until it touches the anterior edge of the centrum 

 and in the lumbers it gradually shortens and finally unites with the face for the capit- 

 ulum of the rib. The articular faces of the zygapophyses are very oblique, not hori- 

 zontal, and the spine rises vertically from directly over the center of the vertebra. 



