io6 AMERICAN PERMIAN VERTEBRATES 



tibial is about one-third that of either side. The popliteal surface 

 is shallowly concave, occupying the lower fourth of the bone. 



The tibia (Plate XII, Figs. 6-n) is a little more than three-fifths 

 the length of the femur, and has a much-dilated proximal extremity, 

 a slender, curved, subtrihedral shaft, and a moderately expanded 

 lower extremity. The distal end is curved backward so that the 

 truncated distal extremity is at an angle of nearly forty-five degrees 

 with the long axis of the bone. The greater diameter of the 

 proximal articular surface is nearly transverse in position in the 

 articulated skeleton. The cnemial elevation in front is not very 

 broad or high, the surface on the outer side of it partly underlying 

 the fibula in articulation; that on the inner side is convex. The 

 distal extremity is subangular on the inner anterior part ; the other 

 sides more rounded. 



The fibula (Plate XIII, Figs, 2, 3), hitherto almost unknown 

 among the Texas reptiles, is a very slender bone, and is distinctly 

 longer than the tibia, the planes of its two extremities slightly twisted 

 on the longitudinal axis at an angle of ten or fifteen degrees. The 

 proximal end is convex on its outer dorsal side, gently concave on 

 the tibial, the proximal articular surface long and narrow, oblique 

 both inwardly and anteriorly. The distal extremity, much the 

 broader, is also convex on the dorsal, concave on the ventral 

 side, with a round distal outline, extended higher up on the more 

 expanded inner side. The articular surface, long and narrow with 

 a slight sigmoid curve, is directed somewhat obliquely forward. The 

 slender shaft is nearly straight in its middle part, or slightly 

 curved in outline. 



Foot (Plate XIII, Fig. i; Plate IV, Figs. 6, 7). The astrag- 

 alus, or tibiale plus intermedium, is subquadrangular in shape, 

 its proximal end, that for the partial articulation of the fibula, 

 the stoutest. The anterior or dorsal surface is concave throughout, 

 with its margins elevated; the posterior or plantar surface is nearly 

 flat above. The fibular surface joins the calcaneal border in a 

 little more than a right angle; this surface is a little broader from 

 side to side than from back to back, is gently concave, with its plane 

 rectangular to the anterior surface. The large tibial articular 

 surface is oblique to the greater plane of the bone, as also to the 



