MORPHOLOGICAL REVISION OF THE SUBORDER. Ill 



The sacrals^ the twenty-eighth to the thirtieth, are abruptly longer than the last 

 lumbars and are closely fitted one against the other, the lower edges being in close 

 contact and the intercentra reduced to small scales firmly coossified with the centra. 

 The transverse processes are reduced to faces only, and the ribs are strongly anchy- 

 losed to the sides of the centra. The head of the anterior rib projects forward beyond 

 the anterior end of the centrum forming a pair of faces which articulate with the 

 posterior edge of the last lumbar. This rib is larger than the others and the distal 

 end is turned downward, forming a broad vertical face which is applied to the inner 

 side of the ilium (plate 17, figs. 3 and 4). The two posterior ribs are smaller and do 

 not have the distal ends turned down to form a vertical face ; they were inclined 

 forward, and as shown in D. dollavianus seemed rather to support the anterior rib than 

 to support the ilium fplate 23, fig. i). The spines of the sacrals are much reduced in 

 height and strength ; the firm union of the centra and consequent immobility of the 

 sacrals has caused the partial atrophy of the zygapophyses so that between the sacrals 

 they are \txy small, even anchylosed, in some specimens. 



The anterior caudals, thirty-first to the thirty-ninth, are all of similar form and 

 the last is as perfect in form as the first, although it is only one-half the size. The 

 rapid reduction in size without any assumption of the cylindrical form characteristic 

 of long-tailed forms indicates the absence of a long tail, though chevrons of good size 

 occur; this is supported by the fact that in none of the numerous specimens of 

 Dimetrodon has any portion of a long tail been discovered. The centra of the caudals 

 are short with nearly circular faces ; the transverse processes are reduced so the ribs 

 are attached directly to the sides of the centra, but not anchylosed in the anterior ones. 

 The faces for the capitulum and tuberculum are separate in the anterior ones, but after 

 the fourth are united and on the ninth are very small. On either side of the bottom 

 line the centrum is marked by a deep pit which throws the mid-line into a rounded 

 ridge ; it will be seen that the most posterior lumbars and the anterior caudals strongly 

 resemble each other. The intercentra of the anterior caudals are flat and wide antero- 

 posteriorly and underlie the point of contact of the two adjacent centra, but between 

 the fifth and sixth the lower edge of the faces of adjacent centra are cut away leaving 

 a wide triangular interspace to accommodate the head of the chevrons. The head of 

 the chevron fits in between the two vertebrae in such a manner that the long terminal 

 process is inclined sharply to the rear. Just below the head the chevron is pierced by 

 an elongate foramen. The ribs are short at best and on the ninth vertebra are reduced 

 to mere stubs. 



The ribs are found on all vertebrae from the atlas to the ninth caudal. The 

 capitulum and tuberculum are well-developed and widely separated, even in the 

 atlantal pair, and only in the reduced ribs of the posterior lumbars and caudals do the 

 two unite. In the cervicals and anterior dorsal the capitulum articulates with a face 

 developed on a strong tuberosity on the lower side of the intercentrum ; Cope regarded 

 this as a characteristic feature of specific value, but it occurs in all members of the 

 genus. In the mid-dorsal region the head of the rib slips from the side of the inter- 

 centrum to the end, between the centra, and articulates with the intercentrum by a 

 cartilaginous connection ; the articular face disappears from the intercentrum in all 

 but D. inacrospondylus, where there is a wide expansion of the ends of the inter- 

 centrum with a face for the rib even in the lumbar region. 



At the eighteenth or nineteenth the capitulum of the ribs suddenly slips from its 

 position between the centra to the edge of the centrum where a face appears for it ; 



