"276 S. W. AVILLISTON^NEW GENERA OF PERMIAN VERTEBRATES 



lower fifth of the bone, and near the middle of the popliteal surface. 

 The shaft of the bone for about the middle two-fifths is very slender, 

 almost cylindrical, save for the crest behind, and is straight. The distal 

 expansion of the bone begins a little above the lower fifth and is a little 

 greater than the proximal one. The lateral linear concavities of the bone 

 on the two sides are nearly symmetrical. The distal articular surface of 

 the bone has sharp borders, indicating a considerable amount of cartilage. 

 The end is much broader transversely than from before backward. The 

 transverse tibial surface looks downward and a little backward and in- 

 ward. The fibular surface is a little longer from side to side and looks 

 markedly outward, backward, and downward. Its width in the inner 

 side is more than one-half of the whole width of the extremity, with 

 narrow extensions both in front and behind. The fibular condylar pro- 

 jection is much thinner and less extensive than the tibial. The extensor 

 groove in front of the distal end is broad and moderately deep, limited 

 on the outer side by a high and rather sharp ridge. On the back side a 

 less prominent, more obtuse ridge opposite the dorsal ridge, and connected 

 with the distal end of the adductor crest, separates a shallow concavity 

 on the inner side from a narrower and deeper one on the outer side. 



Tihia (plate 14, figures 1-4). — The tibia is more than three-fourths 

 the length of the femur. Its upper extremity, as usual, is broadly ex- 

 panded and massive, the lower less expanded and more cylindrical. The 

 upper surface for articulation in the normal position of the bone is broad 

 from side to side and about two-thirds as wide from before back, thicker 

 on the outer than on the inner side, with an emargination on the outer 

 anterior side, the anterior border internally convex, the posterior border 

 nearly straight. The surface is gently convex from side to side, nearly 

 fiat antero-posteriorly, and looks on the whole upward and a little back- 

 ward. The distal articular surface is suboval, its longer diameter run- 

 ning from behind outwardly and anteriorly, with the internal border 

 convex, the outer posterior border more nearly straight or gently convex. 

 The shaft is slender in its middle, broader in section from before back- 

 ward, and is flattened on the inner side. The inner border of the bone 

 is deeply concave, the outer almost straight, save at the lower end. The 

 posterior surface of the bone is flattened on the upper expansion, bounded 

 inwardly by a sharp sinuous crest, which becomes confluent with the con- 

 vexity of the distal extremity. 



Fibula (plate 14, figures 5 and 6). — The fibula is shorter than the 

 tibia, flattened upon its posterior inner surface, and convex from side 

 to side on the opposite. The outer thinner margin is nearly straight to 

 the lower fourth, where it curves inward. The inner border is deeply 



