PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 



55 



extremity of a thin, elongate process directed ventrad and laterad. This surface 

 doubtless corresponds to the outer end of the cartilaginous surface of the proximal 

 end, though separated by a thin, non-cartilaginous interval; and it is very much 

 more protuberant. The trochanter on the tibial side is unusually produced ventrad ; 

 its lower border also is situated further down the shaft, and its extremity is smaller. 

 In other respects this femur does not differ ffom its mate. 



Tibia (fig. 34 a) .• The tibia is rather more than four-fifths the length of the femur. 

 It is a rather stout bone, with its upper extremity much expanded. The articular 

 surface for the tibial condyle is elongate and crescentic in shape, separated by a 

 ridge and groove from the rather large surface of an oval shape which articulates 

 with the posterior side of the fibular condyle of the femur. 



Fibula (fig. 33 F and g) : The fibula is a little longer than the tibia. It has a 

 rather slender shaft, and the lower end is much expanded. The upper end is mod- 



Fig. 34. — Ophiacodon mirus Marsh, X }4- A, left tibia, dorsal 

 surface; B, right pubis, inner surface; C, right foot, dorsal side: 

 D, right tarsus, ventral side. 



erately expanded, with a subcrescentic articular surface 

 placed at an angle of about 60 degrees with the plane of 

 the lower end. Its articular surface fits well the elongate, curved articular border 

 of the femoral condyle, sliding, in articulation, forward and inward and backward 

 and outward; that is, in the extended condition of the leg, the plane of the distal 

 extremity is horizontal, while in the much-flexed condition this plane is turned 

 obliquely. The outer border of the bone is nearly straight, the inner border deeply 

 concave to the lower end. The truncate border is gently curved on the inner side 

 for articulation with the astragalus. 



Foot: The right foot of specimen No. 650 (fig. 34) lay in the matrix with most 

 of its bones in close articulation. The first three digits were doubled under the 

 others, but had all their phalanges attached; one claw, only, was lost in the collec- 

 tion of the specimen. Of specimen No. 651, the left foot was nearly as perfect, the 

 calcaneum and centralia missing. 



The tarsus has the same general structure as have the tarsi of other American 

 zygocrotaphic reptiles, except that there are two free centralia instead of the single 

 one hitherto known. The calcaneum is broad and flat, subcircular in outhne, with 

 its free borders thin. The astragalus is rather more elongate than usual, and its 

 tibial facet is separated rather widely from the fibular one. The perforating fora- 

 men between the two bones opens near the distal end of the articulating borders. 

 The two centralia were found quite in position, as were all the other bones of the 



