124 REVISION OF THE PELYCOSAURIA. 



triangular with the apex downward. Each edge is formed by a supporting ridge as 

 described in Dimetrodon incisivus. The lower ridge is ver)- thin, but reaches from the 

 suture between the neural arch and centrum well down on the centrum, forming a 

 peculiar thin partition between the anterior and posterior portions of the upper part 

 of the vertebra. 



The base of the spine is somewhat flattened. The antero-posterior diameter is 

 24 mm. and the transverse 14 mm. About 70 to 80 mm. up, the spine is somewhat 

 swollen and marked by rugose ridges ; at the swollen portion the spine becomes nearly 

 quadrangular, but with well-developed anterior and posterior grooves ; these continue 

 up the fore and aft faces of the spine (fig. 14). The median line of the lower portion 

 of the anterior groove is marked by a slender ridge from the swollen portion to the 

 upper edge of the neural canal. Above the swollen part, the spine rapidly flattens to 

 a rough figure 00 -shape, with the deeper groove on the posterior side. About the 

 middle of the spine this groove lessens in depth and becomes equal to or even less 

 than that of the anterior side. The apex seems to have been simple. 



A posterior (the seventh j dorsal is longer, and the bottom line is sharp and thin, 

 but the keel is largely lost, for the sides of the centrum rise gradually to the noto- 

 chordal canal, instead of being sharply pinched in just beneath it. The anterior face 

 is nearly circular, with only a slight face below for the intercentnim. The centro- 

 sphenes and centantra are quite good-sized. The neural spine rises from far back, the 

 posterior edge lying almost directly over the posterior zygapophyses. The zygapo- 

 physes are far apart antero-posteriorly, with small and very- oblique faces. The spine 

 is imperfect, but resembled closely that of the last dorsal preserved, the ninth, w-hich 

 is shorter than in the anterior dorsal region and very thin transversely. Both the 

 anterior and posterior edges are thin and prominent at the base; 70 or 80 mm. up, the 

 spine is quadrangular, and then assumes the figure co -shape, but is more nearly round 

 than the anterior ones. 



1l\\^ first lumbar closely resembles the posterior dorsals, except that the anterior 

 edge of the centrum is reverted and widened into a facet for the capitulum of the rib. 

 An intercentrum in position just touches the head of the rib (see the description of 

 the rib articulations below). The bottom line of the centrum is rounded and concave 

 antero-posteriorly ; all trace of the keel has disappeared. 



The posterior himhars are similar in most respects to the same vertebrae of 

 Dimetrodon incisivus. The bottom line is concave antero-posteriorly, but is sharp 

 instead of rounded. The intercentrum is only slightly curved and rather wide antero- 

 posteriorly. It lies below the point of meeting of the two centra, and does not rise on 

 the sides of the centrum above the lower fourth, but touches the lower end of the 

 capitulum of the rib. This spine is weak at the base, but thickens at the upper end 

 to a somewhat clavate form. 



The sacrals are not preserved. 



There are five cmidals^ inclosed in a hard matrix, and not in series. These show 

 a rapid decrease in size, with no tendency to a development of the elongate, cylindrical 

 form indicating a long tail. There are wide intercentral spaces with the chevron 

 bones in place. The chevrons are verj' wide at the top, with two facets on the 

 proximal end almost at right angles to each other. They are both concave from side 

 to side, to fit the corresponding portions of the vertebral centrum. If the length of 

 the chevron is held upright, the anterior face of the two is almost horizontal and the 

 posterior face looks nearly downward. If two adjacent vertebrae are placed in position, 



