124 AMPHIBIA AND PISCES OF THE PERMIAN OF NORTH AMERICA 



side, gently concave anteroposteriorly. The second to the eighth back of 

 the atlas have, on each side, near the upper angle posteriorly, a facet or 

 protuberance for the capitular articulation of the rib. The first hypocentrum 

 back of the atlas is smaller than the succeeding ones; it is shorter and has 

 more acute lateral margins, and is, apparently, without facets for the capit- 

 ular articulations. The second hypocentrum is also somewhat smaller than 

 the succeeding ones, but is provided with a parapophysial protuberance on 

 each side. 



" Pletirocentra: Of the first four vertebrae back of the atlas the pleuro- 

 centra were not recovered; they doubtless had dropped out of their places 

 and were not recognized in the matrix, since a slight depression of this part 

 of the column had loosened all the elements somewhat. However, in the 

 connected series places for them are shown with articulations indicating but 

 little variation in size from that of the following ones. In size the pleuro- 

 centra of the remaining vertebrae are nearly uniform, perhaps slightly longer 

 anteriorly than posteriorly. Each articulates broadly with the posterior 

 side of the pedicle of the neurocentrum, and less extensively with the ante- 

 rior side of the following neurocentrum, as shown in plate i8, and by the 

 narrowed lower extremity with the posterior superior margin of the hypo- 

 centrum, fitting into the angular space between the adjacent neurocentra. 

 The large flat sutural surface for union with the preceding neurocentrum 

 indicates a close, firm union, while that with the succeeding neurocentrum 

 and hypocentrum is more rounded. The pleurocentrum of the first pre- 

 sacral vertebra is narrower than the preceding one. The pleurocentra of 

 the two sides of each vertebra are closely approximated in the middle above 

 a small concavity on the upper side of the hypocentrum, leaving in the artic- 

 ulated parts a persistent notochordal canal. 



"The zygapophyses are stouter in the free or lumbar portion of the 

 column than in that part covered by the carapace, as would be expected, 

 since the comparative rigidity of this part prevents extensive motion of the 

 individual vertebrae upon each other. Their articular surfaces look uniformly 

 upward and inward, and downward and outward, at an angle of something 

 less than 45 degrees. 



"The diapophyses arising from the neurocentra increase rapidly in 

 vertical extent of their rib attachment as far as the eighth, the border con- 

 tinuous with the parapophysial projection on the hypocentra. The ninth 

 suddenly decreases in width, with a wide interval between its lower end and 

 the hypocentrum, which has no parapophysial facet. From the ninth the 

 transverse processes are narrower, with the extremity for rib articulation 

 of but moderate extent. Throughout the series the processes are directed 

 almost transversely outward, with the upper nearly horizontal margin a 

 little thickened and rounded; the upper margin arises a little below the 

 zygapophyses anteriorly, a little lower down posteriorly. The rib margin 

 is straight or gently sinuous from above downward, thinned below and 

 slightly emarginated, ending, as has been described in the first eight, in 

 apposition with the parapophysial facet on the hypocentra. The lower end 

 of the rib margin is considerably in advance of the upper. Beginning with 

 the ninth, where the ribs become single-headed, the transverse processes 

 are of nearly uniform width, the articular surface for the rib placed obliquely 

 to the vertical line. Beginning with the seventeenth vertebra, the first 

 behind the carapace, the diapophyses shorten rapidly, becoming almost 



