258 S. W. WILLISTON NEW GENERA OF PERMIAN VERTEBRATES 



of these expansions^ that of the second vertebra, is small, subtriangular 

 in shape, with rounded corners, and appears not to have been covered by 

 a dermal scute. The second expansion is larger, becoming broader be- 

 hind, and is covered, on its posterior part onl}^, by the first dermal scute. 

 The posterior end of the carapacial series tapers more gradually than 

 does the anterior to a narrower extremity, and like that has, apparently, 

 no dermal plate over the last of the series, the penultimate spinal expan- 

 sion supporting the posterior margin of the last dermal shield. Through- 

 out this series the thinned expansions of the spines above^ anteriorly and 

 posteriorly, touch each, other in the present curved condition of the col- 

 umn. In the most anterior part of the colunm, however, the hypocentra 

 are slightly separated, with one longer interval, producing a slight con- 

 vexity of the series below. It thus seems certain that the position in 

 which the column was found, and which has been retained in the mounted 

 specimen, was a normal one for the living animal; that is, with a gentle 

 convexity antero-posteriorly of the carapace, and a slight concavity below. 

 Nor would it have been possible for the living animal to have fully 

 straightened, out the column without actually dislocating the zyga- 

 pophysial articulations. A slight lateral bending was possible in life, as 

 shown by the position in which the bones were found ; but even this could 

 not have been extensive in the front part, since the free dermal plates 

 which glided smoothly over the fixed spinous expansions would have met 

 each other at their lateral extremities, if the curvature was at all decided. 

 Back of the carapace, however, a greater flexibility was possible, since the 

 zygapophyses here are somewhat larger, and the free spines were sepa- 

 rated above by a greater interval. 



GABAPACE 



(Plate 9 and plate 12, figures 5, 6) 



The dermal plates are of nearly uniform length, increasing slightly in 

 expanse to the middle of the series — that is, at the summit of the dorsal 

 convexity. Each fits accurately and closely over the contiguous borders 

 of the adjacent spine roofs, separated from each other by a space of 1 or 

 2 millimeters. It is evident, from the structure of the spine expansions 

 with the greater beveling in the middle behind, that the chief motion was 

 at the anterior part of each dermal plate. The upper surface of these 

 plates is slightly irregular, with shallow depressions or pits, the margins 

 in front and behind parallel, with a slight obliquity backward; their 

 outer angles are slightly rounded, and their outer, thin borders are nearly 

 straight, or with a slight emargination. Each of these dermal plates is 

 composed of two elements, a median longitudinal suture being evident in 



