86 REVISION OF THE PELYCOSAURIA. 



large ribs with widely divided capitulum and tiiberculum ; the distal end is gone from 

 all, but enough remains to show that they were quite broad distally. 



Tivelfth to seventeenth vcrtebi'CB : These are ver}- similar to the preceding; the 

 centrosphene and centantra are larger than in the cervicals, but are still very small. 

 Between the seventeenth and eighteenth there is a small intercentrum, narrow antero- 

 posteriorly, with sharp ends not showing any face for the capitulum of the rib. 



Eighteenth to twenty-first vertebrce : The centrum of the eighteenth has a sharp 

 ridge on the mid-line below, but the median groove is lost. The diapophysis is short 

 and inclined a little backward so the articular face looks slightly backward as well as 

 outward. The diapophysis lies far forward almost directly beneath the anterior zyga- 

 pophysis and the lower ridge reaches quite to the anterior edge of the centrum and has 

 a flat articular surface on its edge showing that the capitulum and tuberculum of the 

 rib are fast approaching each other. On the twenty-first the lower ridge has entirely 

 disappeared as a ridge and the diapophysis is a short stub with its articular face look- 

 ing directly out. 



Twenty-second to twenty-fifth vertebrce : On the twenty-third the rib is fused with 

 the centrum. It is verj' thin and slender at its proximal end, differing in this respect 

 verj' markedly from the condition of Dinictrodon. From this point the ribs are traceable 

 to the sacrum, but are very small and almost rudimentary. On the twenty-fourth a 

 longitudinal ridge appears on the side of the centrum, on the succeeding one it is 

 stronger and remains until the twenty-ninth, where it is suddenly absent. The sud- 

 den appearance and short duration of this character is of great interest, as it was on 

 an isolated vertebra of this region that Cope founded the species Tlieropleiira trian- 

 gulata. The bottom line is a flattened keel. 



Twenty-sixth to thirtieth vertebrcB : These vertebrae immediately preceding the 

 sacrals are slightly, but not greatly, shorter than the rest of the vertebrae. The bottom 

 line is very concave and the keel is reduced to a low obtuse ridge. The ribs are very 

 small, but it looks as if on the thirtieth there was a small opening between the capitulum 

 and tuberculum and the side of the centrum. 



Thirty first and thirty-second vertebrcs : These are the sacrals. They are not con- 

 nected with the anterior series, but not more than one or two vertebrae, if that many, 

 are lost. The sacral ribs are large and strong and united solidly with the sides of the 

 centrum. The two vertebrae are firmly connected so that the posterior and anterior 

 zygapophyses of the first and second sacrals are greatly reduced and it seems as if the 

 neural arches of the two were even united by anchylosis. 



Thirty-third to forty-first vertebrce: The nine caudals connect with the two 

 sacrals completing the posterior series. The thirty-third, first caudal, has a well- 

 developed rib still attached. The rib is much larger than those of the lumbar series, 

 the capitulum and tuberculum are distinct, but close to each other, and the capitulum 

 is attached to the anterior edge of the centrum. The neural arch is quite high ; the 

 sides of the centrum are concave antero-posteriorly, but convex vertically. The 

 median line below is a blunt keel, but as the sides of the centrum are convex vertically, 

 there are lacking the deep pits on either side of the mid-line of the centrum below, so 

 characteristic of the caudals of the Clcpsydropida-. 



The second caudal has lost the rib and the faces for the capitulum and tubercu- 

 lum of the rib are shown. These are large flat facets not elevated on a process, with 

 their long axes at right angles and separated by a very narrow space. The face for 

 the tuberculum is on the upper edge of the centrum and the face for the capitulum is 



