136 REVISION OF THE PELYCOSAURIA. 



the fore-and-aft edges are drawn oiit into very thin and prominent ridges ; above the 

 rugosity the section of the spine is rounded with grooves on the anterior and posterior 

 surfaces, and its surface is smooth as in Dinietj-odon macrospoiidyliis. 



The tenth vertebra shows a slight decrease in the sharpness of the keel ; the 

 anterior end of the keel is rather rounded and the posterior end is double, having a 

 slight narrow groove dividing it longitudinally. 



The eleventh and twelfth vertebrce are longer and the keel below is less strong ; 

 the sides of the centra are regularly convex, the sudden narrowing just below the 

 notochordal canal so characteristic of the dorsals having disappeared. The anterior 

 face is nearly as round as the posterior, due to the loss of the face for the intercentruni 

 on the lower edge. Looking at the vertebra from above it is seen that four strong 

 ridges run from the base of the spine, one to the base of each of the zygajDophyses ; 

 this is true of the same region of all the species of the genus, but in none is it so 

 apparent as in this specimen. 



The thirteenth to the sixteenth are connected. These are rather longer than the 

 anterior dorsals, and the bottom line has a very low keel. The ends of the centra are 

 rather abruptly flared, so that the vertebrce are concave from end to end on both sides 

 and the bottom line. The inferior supporting ridge of the transverse process curves 

 gradually forward in these vertebrae; by the sixteenth it reaches nearly to the anterior 

 edge of the centrum. 



The seventeenth to the twentieth are imperfect, but show the gradual elongation 

 forward of the inferior supporting ridge of the transverse process and its growth in 

 height; in the twentieth it is nearly as high as the transverse process itself. The 

 elevation of this ridge is accompanied by the development of a pit both behind it and 

 above it. On the nineteenth the ridge has reached the anterior edge of the centrum, 

 and on the twentieth the face for the capitulnm appears on the edge of the centrum. 

 The centra grow more elongate and the keel becomes lower, but does not disappear. 

 Between the eighteenth to the twentieth there is a small intercentruni with sharp 

 ends, lacking an articular face, as in Dimetrodon incisivns. 



The twenty-first to the twenty-third vertebrce : The first of these is imperfect and 

 may be the posterior half of vertebra twenty. On the twenty-second the ridge from 

 the transverse process just touches, but does not join, the capitular face. On the 

 twenty-third the two are united. 



There are two anterior caudals in the box with these vertebrae, but they seem 

 entirely too large to belong to the specimen. 



No. 1060 University of Chicago: The axis. The upper portion of the spine is 

 broken away, but the lower part shows that it was enlarged, having much the same 

 form as No. 114 University of Chicago. The centrum is relatively much more elon- 

 gate than in Dimetrodon incisivns. The lower surface is marked by a sharp but verj' 

 low ridge, which does not reach to the anterior and posterior edges of the centrum. 

 The under side of the two anterior zygapophyses is concave, so that as they meet they 

 form an arch over the neural canal and extend well anterior to the edge of the cen- 

 trum ; at the base of the zygapophyses are well-developed ceutantra. The diapophyses 

 slant backward, but are not at any point free from the centrum, and do not extend 

 below its middle. The anterior face of the centrum has a well-developed face for the 

 intercentrum, occupying nearly one-third of the height of the face. The posterior 

 face is circular. 



