POPOSAURUS GRACILIS, A NEW REPTILE 521 



The iirst caudal differs from the second in that the centrum 

 of the former is somewhat longer, has larger articular faces, and 

 has the chevron articulation at the posterior end only. There is 

 also a considerable difference in the diapophyses of the two verte- 

 brae. In the first they are broad and, although badly weathered, 

 suggest a lateral face of perhaps 30 mm. anterior-posterior extent as 

 compared with 10 mm. in the second. In the first caudal the 

 diapophyses are placed farther forward and slightly lower than in 

 the second and are slightly supported by the centrum. 



The condition of the leg bones is such that little can be done 

 in the way of description other than stating approximate measure- 

 ments. The femur is at least 465 mm. long and very slender. In 

 an uncrushed condition the shaft was probably about 45 mm. in 

 diameter. The head is considerably expanded antero-posteriorly, 

 measuring 85 mm. in that direction. Little can be said of the 

 condyles except that the antero-posterior extent of the articular 

 face was great, perhaps 85 mm. or more. 



A piece of the tibia, 125 mm. in length, a portion of the shaft 

 near the proximal end, remains imbedded in the matrix in its 

 natural relation with the left femur. In life the shaft was probably 

 about 30 mm. in diameter. The left fibula, some 345 mm. in 

 length, is very thin laterally and much expanded antero-posteriorly 

 at both the proximal and distal ends. The shaft, which is 40 mm. 

 in width, is regularly concavo-convex antero-posteriorly throughout 

 its entire length, the convexity outward. Were this a feature due 

 to distortion only, one would hardly expect the solid articular ends 

 to conform to the curvature of the thin shaft as they do. Further- 

 more, polished cross-sections of the shaft do not suggest con- 

 siderable flattening of its walls. 



RELATIONSHIPS 



The specimen has little in common with the phy tosaurs ; besides 

 its apparently perforate acetabulum, of which there is but the 

 slightest suggestion in some of the phytosaurs, it shows a marked 

 difference in the shape of the ilium and in the comparatively large 

 number of vertebrae fused together in the sacrum. In the phyto- 

 saurs there are but two sacrals and these are free. As Dolicho- 

 brachium, described by Dr. S. W. Williston from the Trias of 



