262 S. \V. WILLISTON NEAV GENERA OF PERMIAN VERTEBRATES 



12^ figure 5^ and by the narrowed lower extremity with the posterior 

 superior margin of the hypocentrum, fitting into the angular space be- 

 tween 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 presacral 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 articulated 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 ver- 

 tical 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 articu- 

 lation 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 trans- 

 verse 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 sessile in the last two, in which the rudimentary ribs 

 seem to be anchylosed to their extremities. 



ATLA8 



The atlas was preserved in this specimen in place. It is somewhat 

 eroded and does not seem to differ from a better preserved specimen be- 

 longing to the closely related genus Dissorophus, which T have figured in 



