122 AMERICAN PERMIAN VERTEBRATES 



indication in any of the recovered specimens of ventral ribs, such 

 as occur so commonly in Varanosaurus and other genera of the 

 Poliosauridae, and it is assumed that were they existent some 

 indications of them would have been found in the various specimens. 



Sacrum (Plate XVI, Fig. i; Plate XX, Fig. 6; Plate XXIII, 

 Fig. 6). — The sacrum of Casea is composed of three vertebrae, 

 differing from those immediately preceding and succeeding chiefly 

 in their short and expanded ribs. These vertebrae articulate 

 freely with each other, and have free, though small, intercentra 

 below. The centra are convex from side to side below, compressed 

 on the sides. The spines are proportionately a little stouter 

 than those immediately preceding the sacrum, their upper extremity 

 nearly as broad as long. The ribs are attached quite like the pre- 

 ceding ones, but are stouter, and like the three or four succeeding 

 pairs, by a short head reaching below the middle of the centrum 

 and articulating in part with the preceding centrum across the 

 intercentral space. The tubercular part is very stout and heavy, 

 extending high up on the arch. Between the head and tubercle 

 the small foramen is persistent, as in the posterior lumbar vertebrae. 

 The shaft of the ribs is subcylindric or prismatic, dilated at about 

 the middle part into a broad flattened plate, broadly convex out- 

 wardly and curved downward so that its lower border is about 

 on a level with the lower margin of the centra. The expanded ends 

 of the first and second pairs are about equal in size; that of the 

 third is smaller, but little more than half that of the first. It is 

 stouter at its extremity and does not descend quite as far. The 

 extent of these three ribs appears to be slightly greater than the 

 greatest extent of the ilia antero-posteriorly. Their union with the 

 ilia was a comparatively loose one, chiefly ligamentous, the ends of 

 the ribs merely touching the ilia, which show no sutural markings 

 for their union. 



Caudal vertebrae (Plate XVI). — No single tail yet recovered is 

 quite complete. That of specimen No. 655 comprises seven verte- 

 brae, as figured in Plate XVI, Fig. 1 ; that of No. 656 comprises 

 about twenty in a continuous series with the sacrum and complete 

 precaudal series; the incomplete tail of No. 657 has not been freed 

 from the matrix, but has twenty-two vertebrae in the series con- 



