MORPHOLOGICAL REVISION OF THE SUBORDER. 85 



Atlas : The centrum of the atlas is lost, but the neural arch of one side clings 

 to the under side of the skull ; it is \^xy similar to the neural arch of Dimetrodon 

 (plate 16, figs. 3 and 4). 



Axis : The centrum of the axis is somewhat elongate and the bottom line forms 

 a sharp ridge. The sides are concave and marked below the transverse process by 

 several longitudinal lines. Just below the transverse process of the left side is a good- 

 sized nutrient foramen. The transverse process springs from the neural arch and is 

 very short. The articular face is small and the distal end of the process is not free 

 from the side of the centrum. The spine is rather high, thin, and expanded antero- 

 posteriorly, with a wide superior edge. The anterior zygapophyses are small, but not 

 so disproportionately small as in the ClepsydropidcB. Centrosphene and centantrum 

 (see morphological description of Dimetrodon incisivus) are present, but small. 



Tkiid vertebra (counting the atlas as first) : The bottom line of centrum has 

 a distinct low keel, but the longitudinal lines on the sides are less definite. The dia- 

 pophysis is longer than in the axis and inclined more sharply to the rear. The lower 

 portion of the anterior face of the centrum is slightly cut out to accommodate an inter- 

 centrum. The spine is of nearly the same shape as in the axis. Several small two- 

 headed ribs, which probably belong to the anterior cervicals, cling to the side of the skull. 

 Fourth to seventh vertebrcB : In these the diapophyses grow gradually shorter and 

 stand out more nearly straight from the centrum. They do not originate, as in 

 Dimetrodon^ in three processes, one each from the anterior and posterior zygapophysis 

 and one from below, but rise directly from the neural arch below the line of the zyga- 

 pophyses. An inferior supporting ridge is present, 

 which at first inclines backward, but soon changes so 

 that on the fifth and sixth vertebrae it inclines forward 

 toward the anterior edge of the centrum. The bottom 

 line has now a distinct keel, but is formed by a gradual 

 contraction of the sides of the centrum (fig. 24) and 

 not by a sudden pinching in of the sides immediately 

 below the intercentrum as Dimetrodon (fig. 25). 



Eighth to eleventh vertebra- : In these the bottom 

 Fig. 24.-Cros5.secuon through middle of jj^^ j^ shorter than thosc anterior to it and the sides 



a dorsal vertebra ot I heropleura, i 1 i j.1. • i i. _ -u-.j. 



. ■ 11 , ^ , J are somewhat compressed below the mtercentrum, but 



showing gradual contraction of sides "^'^ ouiii^ vviicii. v.v./iii^i^.^ , 



below notochordal canal. X H- nothing like the Condition of Dimetrodon. The lower 



Fig. 25.— Cross-section through middle ot line is a broad roundcd keel with a median longitudinal 



a dorsal vertebra of Dimetrodon, gj-Qove that uiakcs it appear doublc ; the median groove 



showing the sudden pinching in of j^ narrowest at the middle of the centrum and widens 



the sides below the notochordal canal , ., , ^tm • 1 i- . i i i_ 



formmg the narrow keel. toward each end. The sidcs of the vertcbrcE are con- 



cave so that the edges of the articular surfaces are very 

 prominent. This is true of all portions of the column posterior to the eighth vertebra. 

 Tbe diapophyses stand out straight from the upper edge of the centrum and are very 

 short ; the articular face looks directly out. The lower supporting ridge extends for- 

 ward toward the anterior edge of the centrum, but does not reach it in any of this 

 group of vertebrae. The change in position of the lower supporting ridge is peculiar 

 in that it is entirely independent of the diapophysis proper ; this shows little change 

 of form or position in the anterior two-thirds of the column. There are two or three 



