CACOPS ASPIDEPHORUS 261 



a median prominence. The expansions are sometimes large, resembling 

 the dermal bones of the crocodiles, and in that case the median promi- 

 nence is a keel. On the smaller expansions the latter is a mere apex. 

 There are narrow, flat bones which I suppose to be neural spines, which 

 are ornamented with innosculating ridges." ^ These characters agree 

 better with a spine figured and described by Case as Zatracliys crucifer 

 Case,^ the type of which I have examined. This is of much hirger size, 

 and is in the shape of a cross, the median projecting piece above and the 

 upper surface of the transverse arms are deeply pitted — a radically dif- 

 ferent type from that of either Aspidosaurus or Cacops. The description 

 of the skull seems to preclude the possibility of the genotype being con- 

 generic with Cacops aspidephorus. 



BTPOCENTRA 



(Plate 9, plate 12, figure 7, and plate 14, figure 12) 



The hypocentra, like the arches, are of nearly equal size and extent 

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

 concave antero-posteriorly. The second to the eighth back of the atlas 

 have, on each side, near the upper angle posteriorly, a facet or protuber- 

 ance for the capitular articulation of the rib. The first hypocentrum 

 back of the atlas is smaller than the succeeding ones ; it is shorter and 

 has more acute lateral margins, and is, apparently, without facets for 

 the capitular articulations. The second hypocentrum is also somewhat 

 smaller than the succeeding ones, but is provided with a parapophysial 

 protuberance on each side. 



PLEUROCENTRA 

 (Plate 9 and plate 12, figure 5) 



Of the first four vertebrae back of tlie atlas the pleurocentra were not 

 recovered; they doubtless had dropped out of their places and were not 

 recognized in the matrix, since a slight depression of this part of the 

 column had loosened all the elements somewhat. However, in the con- 

 nected series places for them are shown with articulations indicating but 

 little variation in size from that of the following ones. In size the pleuro- 

 centra of the remaining vertebrae are nearly uniform, perhaps slightly 

 longer anteriorly than posteriorly. Each articulates broadly with the 

 posterior side of the pedicel of the neurocentrum, and, less extensively, 

 with the anterior side of the following neurocentrum, as shown in plate 



s Proceedings of the American Philosophical Society, vol. xvil, 1881, p. 523. 

 ' Journal of Geology, vol. xi, p. 399. 



