OSTEOLOGY OF AMERICAN PERMIAN VERTEBRATES 389 



that I have figured, are very nearly complete. In Fig. 5, E, the 

 two bones are shown as completed from these and other specimens, 

 aided by the complete specimens of a little smaller size shown in 

 the associated skeleton of Figs, i and 2. The tibia (Fig. 5, E-H) 

 is considerably expanded above, and has a prominent cnemial 

 ridge ending in a rounded, rugose, cnemial crest above, for the 

 attachment of what was evidently a strong quadriceps extensor 

 ligament. Its upper surface is gently concave and fits very per- 

 fectly the distal articular surface of the femur. Its shaft is nearly 

 straight and nearly cylindrical, or a little flattened. Its lower 

 extremity is but little dilated, and its end is squarely truncate. 

 The fibula is a Httle more curved, but scarcely differs in size from 

 the tibia. Its upper end is moderately dilated on an oblique 

 plane. Its lower extremity, as shown in Fig. 5, yl , is nearly cylindri- 

 cal, its articular surface distinctly divided into two facets, the one on 

 the tibial side for articulation with the astragalus, a little narrower 

 than the calcaneal, and rounded. 



Foot. — The tarsus (Fig. 5, /, K), more or less complete, is pre- 

 served in two specimens, and in part in a third. Of one of these 

 the bones are shown as they lie at the ends of the tibia and fibula 

 in Fig. 5, B, and as brought as nearly as possible into their articular 

 relations in Fig. 5, /. The tibia in this illustration is shown partly 

 from the side and has apparently undergone a slight external curva- 

 ture, though it is possible that this curvature is natural, and that 

 I have represented the shaft in Fig. 5, E, too straight on the lower 

 part. The astragalus is a small cuboid bone, very different in 

 form from that of other known Permian reptiles, in that I can detect 

 no surface on the inner side for the arthrodial articulation of the 

 tibia. Its articulation with the tibia seems to have been firm and 

 close, with not much motion. Lying at the outer side of this bone 

 is a smaller one which can be interpreted only as a centrale of small 

 size, articulating with the third and fourth tarsalia and probably 

 somewhat with the astragalus. This is a rather unusual position 

 for the centrale, but it seems certain that such were its relations. 

 The first two tarsalia are relatively large, the third and fourth 

 small. The fifth tarsale and the calcaneum are missing in this 

 specimen. 



