MORPHOLOGICAL REVISION OF THE SUBORDER. 



107 



The vertebral column (plate 13, fig. 2) : The cohimn contains twenty-seven pre- 

 sacrals, three sacrals (No. 4008 Am. Mus.), and at least uinecaudals(No. 4040 Am. Mus.). 



The atlas (Nos. i and looi University of Chicago) : Preceding the atlas is a 

 preatlantal intercentrum ; it is larger than the second, atlas-axis one, and has well- 

 developed tuberosities which carry faces for the head of the atlantal rib. The anterior 

 face is the larger, is concave vertically as well as transversely, and lodges the hemi- 

 spherical occipital condyle. The posterior face is convex vertically and fits against the 

 saddle-shaped lower portion of the anterior articular face of the atlas. 



The centrum of the atlas is broad above antero-posteriorly and contracted below. 

 The shape of the face is best seen in plate 16, figs. 5 and 6. It is elongate, heart- 

 shaped, and partly divided into two parts by a compression near the middle ; the 

 lower part of the face is distinctly saddle-shaped. The foramen for the exit of the 



Fig. 32. — First intercenhum, atlas, second intercentrum, and axis of D. incisicus. No. 

 University of Chicago. /«/<■?-/, preatlantal intercentrum, a/, atlas centrum ; inter z, 

 second intercentrum. X Yz- a. Anterior face o( atlas. 



Fig. 33. — First intercentrum, atlas, second intercentrum, and axis ot D. incisious, No. 1001 

 University o( Chicago. /«/{■;/, preatlantal intercentrum ; «^, atlas centrum ; inter 2, 

 second intercentrum. X Yi- tr. Anterior lace ot atlas. 



Fig. 34. — First intercentrum, atlas, second intercentrum, and axis of D. g/gas. No. 1002 

 University of Chicago, inter i, preatlantal intercentrum ; at, atlas centrum ; inter 2, 

 second intercentrum. X Y- a. Anterior face of atlas. 



Fig. 35. — Axis of D. doUovianus, No. 1 14 University of Chicago. X Y- 



notochord is near the middle of the anterior face and is very 

 small as compared with the deep funnel for its entrance on the 

 posterior side. There is in the University of Chicago collection 

 an atlas firmly fused with the preatlantal intercentrum. 



The neural arches of the atlas (No. lOOi University of 

 Chicago) are semicircular flattened elements entirely free from 

 the centrum. In the photograph, plate 15, fig. 2, they are placed as they lay on the 

 ground with reference to the other vertebrae (one has been turned over to show the oppo- 

 site side;. On what is probably the outer surface there is a well-developed flattened 

 surface near the middle which seems to have articulated with the prezygapophyses of 



