CACOPS ASPIDEPHORUS 257 



the removal of the thin inerusting matrix, however, a few of the pleuro- 

 centra of the anterior vertebrae and some of the small elements of the 

 tail were lost, notwithstanding the most scrupulous care. Of these, only 

 the possible pleurocentra of the atlas and the dorsal elements of the first 

 5 or 6 caudal vertebrae have any morphological significance ; future prepa- 

 rations of other skeletons yet contained in the matrix will doubtless com- 

 plete even these small details. The column as found was continuous from 

 the skull to the tip of the tail, without break, save that the last few caudal 

 vertebrae were slightly angulated from the rest of the series. The pre- 

 sacral vertebrae had a gentle, sinuous curve, with the convexity to the 

 right as far back as the end of the dermal carapace, to the left from 

 thence to the sacrum. There is also a slight vertical sinuosity in the same 

 regions, convex above anteriorly, below posteriorly. These curvatures 

 seemed so normal that no attempt has been made to reduce them in the 

 mounted skeleton, and I have figured the column as it lay. A slight 

 pressure to the left has crowded the ribs upward on that side and down- 

 ward on the opposite side, but to a very slight extent only. The perfect 

 union of the different elements, at least as far as the beginning of the 

 chevron caudal vertebrae, removes all possible doubt as to their number 

 and relations — 21 presacral, 2 sacral, 6 pygals, and 15 or 16 chevron 

 caudals. The spines, save those of the first 2 vertebrae, are of nearly 

 uniform length throughout the carapacial series, 15 in number, a trifle 

 longer perhaps in the anterior and middle region, and a little more slen- 

 der in the last 3 or 4. Those of the free presacral vertebrae are progress- 

 ively shorter and less stout. Throughout the series covered by the cara- 

 pace they are slightly thickened at the upper end, with the anterior and 

 posterior margins thinned, and with a lateral ribbed thickening on each 

 side near the middle, as though for the support of the terminal expansion. 

 Covering the top of each spine there is a roofiike expansion, wider in 

 the middle and narrowed at each lateral end. Their sides slope down- 

 ward at an angle of about 45 degrees to the full width of the superincum- 

 bent dermal scutes. The anterior margin of these plates is uniformly 

 beveled for articulation with the posterior margin of the superincumbent 

 intercalated dermal scute. The posterior beveling is much broader in the 

 middle. Presumably these expansions are outgrowths from the top of 

 the spine, cartilaginous in origin, but of this I do not feel entirely as- 

 sured, since in every case where I have removed them I have found what 

 appears to be a sutural 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 



