264 S. W. WILLISTON NEW GENERA OF PERMIAN VERTEBRATES 



vertebrse composing it. And this is one of the characters which have been 

 urged as indications of direct genetic relationship between the micro- 

 saurs, in which two sacral vertebrae are known to occur in some forms at 

 least, and the reptiles. Hitherto, not only among recent, but also among 

 extinct, amphibians, excluding the Microsauria, but one sacral vertebra 

 has been known to occur, though I find certain references to sacra with 

 two vertebrae among the older writers on the stegocephs. In Cacops we 

 have two well developed pairs of sacral ribs broadly attached to the ilia. 

 Of these, the first pair is a little larger and stouter than the second, 

 though differing otherwise but slightly. The stout vertebral ends have 

 two articulations, with a small non-articular surface between them, the 

 upper and larger one attached firmly to the neurocentrum, the lower to 

 the upper border of the hypocentrum, which again presents a parapo- 

 physial protuberance for its union. The somewhat crushed condition of 

 the arches of the sacral vertebrae, as they were found lying in the pelvis, 

 prevents the determination with certainty of the relations of the neuro- 

 centra to the hypocentra, but I suspect that they articulate on the sides 

 with the ribs only and not with the hypocentrum. Beyond the articular 

 head the stout shaft of the ribs is constricted for a short distance into an 

 oval form, and then suddenly expands into the large flat or outwardly 

 concave portion for union with the ilium. This thinned expansion of 

 the first rib has an emargination on the upper posterior border, in which 

 fits loosely the lower anterior border of the second rib, the two forming 

 an elongated, nearly plane surface, which extends the whole length of 

 the lower part of the ilium in its greatest width nearly opposite the upper 

 part of the acetabulum. The first sacral hypocentrum is rather larger 

 than the preceding one, with a parapophysial facet at each side for the 

 rib. The second hypocentrum, of nearly equal size, has also a like facet 

 for the rib on each side. The neurocentra of these two vertebrae are in 

 part missing, apparently due to some accident before fossilization. 



TAIL 



The tail was preserved complete, but the flattened end was slightly 

 disclocated, doubtless due to its thin, compressed form. The small bones 

 of the neural side of the first six, or pygal, vertebrae were so small and so 

 confused in the matrix that not much could be made of them. The six 

 pygal hypocentra were found attached in a continuous series. There is a 

 slight, very slight, possibility than an additional one may have been lost 

 in the matrix at the place where the dislocation occurred, but I think not. 

 The tail could not have been more than a fourth of an inch longer or 

 shorter than is shown in the restoration and plate. The first six, or 



