MORPHOLOGICAL REVISION I23 



surface, and the top of the spines is rounded on the margin, with an extrant 

 angle between it and the plate. The surface of these expansions is smooth, 

 both above and below. The narrowed outer extremities are either rounded 

 or with a slight emargination. The first 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 expan- 

 sion is larger, becoming broader behind, and is covered, on its posterior 

 part only, 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 expansion supporting the posterior margin of the 

 last dermal shield. Throughout this series the thinned expansions of 

 the spines above, anteriorly and posteriorly, touch each other in the pres- 

 ent curved condition of the column. In the most anterior part of the 

 column, however, the hypocentra are slightly separated, with one longer 

 interval, producing a slight convexity 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 anteroposteriorly 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 zygapophysial 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 

 separated above by a greater interval. 



" Carapace: 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 bor- 

 ders of the adjacent spine roofs, separated from each other by a space of i 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 ele- 

 ments, a median longitudinal suture being evident in many of the shields. 

 In the middle of each above there is a shallow groove bordered on each side 

 by a slight elevation. 



" In the structure of the dorsal shield or carapace the genus is identical 

 with Dissorophus, save that the carapace of the latter genus is very much 

 more extensive, covering practically the whole of the dorsal side of the ani- 

 mal and probably extending further back. Furthermore, in Dissorophus 

 the anterior shield is very large, covering several vertebrae, very much like 

 the anterior shield in certain armadillos. 



"Hypocentra: The hypocentra, like the arches, are of nearly equal size 

 and extent throughout, rather strongly and smoothly convex from side to 



