io8 



REVISION OF THE PELYCOSAURIA. 



the axis. On the opposite, inner, side is a saddle-shaped articular face wider anteriorly 

 than posteriorly. Below this saddle face is a deep pit which communicates by a notch 

 on the anterior edge with a deep groove below the articular face on the outer side. 



On the outer side of the lower end there is on some atlas arches preserved in the 

 American Museum a small smooth facet ; this may have marked the articulation with 



the centrum. The arches were very loose in 

 ^ ^ their position and it has been an almost unan- 



swerable puzzle to place them in any position 

 that would seem the correct one; they were 

 probably held free by a considerable mass of car- 

 tilage and ligament. (Plate i6, figs. 3 and 4.) 



36 37 38 39 



Fig. 36.— Third cervical of D. giganhomogenes. No. 112 University of Chicago. X )4. 

 Fig. 37.— Third cervical of D. gigas, No. 1002 University of Chicago. X '/z. 

 Fig. 38.— Third cervical of D. incisicus, No. 1001 University of Chicago. X /4. 

 Fig. 39.— Third cervical of D. dollovianus, No. 114 University of Chicago. X }i. 



The second intercentrum is smaller than the first, but has distinct processes and 

 facets for the head of the axial rib. 



The axis (No. i and No. looi University of Chicago) has the centrum somewhat 

 elongate with the beginning of a keel on the median line below. The posterior face of 

 the centrum is lower than the anterior face, so that a line drawn through the middle 

 of the centrum antero-posteriorly would be inclined downward toward the rear. The 

 transverse process rises from the side of the neural arch and curves out and downward, 

 at the same time slanting to the rear. The process is not free from the centrum even 

 at its distal end, being bound to it by a strong ridge below. The posterior zygapo- 

 physes are large and well developed, but the anterior ones are very small with flat 

 horizontal articular faces (No. lOoi University of Chicago). The spine is very wide 



