132 



REVISION OF THE PELYCOSAURIA. 



No. 4012 Am. Mus.: The first cervical preserved, the third (?) (described by Cope 

 as the axis) has the same upright form as Dimetrodon incisivus. The centrum is rather 

 higher than wide, so the articular faces are narrowed. 



The next two and a half vertebrte are connected and belong in the anterior dor- 

 sal series ; the>' have the same form as vertebne of the same region in Dimetrodon 

 incisivus ; the lower edges of the centra are recur\'ed to accommodate the intercentrum 

 and rib, but not nearly so much as in Dimetrodon incisivus and Dimetrodon dollo- 

 vianus, because the intercentra have already assumed the characteristic form of the 

 species and the ends project beyond the edges of the centra with distinct terminal 

 facets for the capitulum of the rib. These vertebra do not connect with the series 



which follows, which begins with a half 

 vertebra and contains nine whole ones of 

 the posterior dorsal and lumbar regions. 

 The anterior ones have a low keel on the 

 mid-line of the centrum, but by the fifth of 

 the series the bottom line is rounded ; on 

 the same vertebrte is the first evidence of 

 a broadening of the edge of the anterior 

 articular face to form the facet for the 

 capitulum of the rib. The advent of this 

 face is much more gradual than in Dime- 

 trodon incisivus, where it appears without 

 warning. On the sixth and seventh the 

 face is plain and the lower supporting ridge 

 of the transverse process extends forward 

 almost to the capitular face ; on the ninth 

 of the series the two facets have com- 

 pletely fused. Intercentra are present 

 and show the characteristic form of the 

 species ; the best view is from below. The 

 intercentra are of nearly equal width in 

 the whole series ; slightly concave on the mid-line, but projecting bejond the edges 

 of the centra and the ends enlarged and flattened to accommodate the face for the 



capitulum of the rib. 



After this series are three lumbars, two free and one attached to the sacrum ; 

 they have the shortened, wider form of the posterior lumbars of Dimetrodon i/icisiz^us. 

 The ribs are fused to the sides of the centra, but the capitula project forward beyond 

 the edge of the anterior articular face and into the intercentral space in a manner not 

 seen in Dimetrodon incisivus. In these the intercentra have lost the wide-ended form 

 and underlie the centra as in other species of the same genus. 



The sacrals are of the same form as Dimetrodon incisivus. Between the last 

 sacral and the first caudal is a very narrow intercentrum, but between the first and 

 second caudals the intercentnun is much wider. 



No. 1019 University of Chicago (plates 25 and 26) : The series consists of three 

 connected posterior dorsals, a broken and disconnected vertebra, and then five more in 

 connection. The anterior four have the spines attached, but in the posterior ones the 

 neural arches are broken and the spines are arranged by their characters. The only 

 spine perfect to the tip is one of the lumbars. All of the spines are somewhat twisted 

 and bent, but this seems to be due rather to the accidents of fossilization than to any 

 natural condition. 



58 59 



Fig. 58.— Axis of D. macrospondylus. Nos. 4054, 4055 

 Am. Mus. cs, centrosphene. X %. Anterior view. 

 Fig. 59. — Lateral view o( the vertebra shown in fig. 58. 



