56 



PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 



tarsus save the first two tarsalia, which, with their respective toes, had been pulled a 

 little distance away from the others. They articulate between the astragalus and the 

 first two tarsalia and have their free surfaces in front much less extensive than those 

 behind, indicating the possibility of considerable over-extension of the inner toes and 

 corresponding tarsalia upon the proximal ones. The fourth tarsale is, as usual, con- 

 siderably the largest, and the fifth is the smallest. In fig. 34 c the tarsus is shown 

 in the oblique position of life; in fig. 34 D it is shown from behind, lying horizontally, 

 the surface somewhat convex. This convexity is that in which the bones were found 

 in the matrix, and the position is that assumed in articulation; evidently it was the 



normal articulation of the foot in life, and it is doubtful 

 whether the foot could have been straightened out in the 

 same plane with the bones of the leg. 



The curvatures of the toes, as shown in fig. 34 c, are 

 quite those which the metatarsals and phalanges assume 

 in close articulation with each other. The first meta- 

 tarsal is, as usual, short and broad; its proximal articu- 

 lar surface is of considerable extent and is convex, both 

 dorsoventrally and transversely, showing conclusiveh- 

 that the toe was capable of much over-extension as well 

 as of much free lateral movement. On the other hand, 

 the third and fourth metatarsals, of much greater length, 

 fit closely at their proximal ends with their respective 

 tarsalia, and were not capable of much movement, either 

 laterally or dorsoventrally. The second metatarsal, of 

 considerably greater length than the first, had also much 

 freedom of movement in both directions, as had also 

 the fifth metatarsal. The distal end of the astragalus 

 has a much greater extent of surface than the sum of 

 the opposing surfaces of the centralia, indicating a con- 

 siderable arthrodial action between them. The ungual 

 phalanges are more nail-like than claw-like; they are 

 thin and flattened, and even somewhat dilated at their 

 tips. They are scarcely curved and are not pointed. 



The foot was broad and flat, one well adapted for 

 soft or imeven ground, but quite unfitted for climbing 

 or running. The posture of the leg shown in the restor- 

 ation is clearl}' a natural one. The articular surfaces are 

 sharply and clearly defined. 



This tarsus of OpJiiacodoii, the most generalized 

 known among reptiles, may be compared with that of Trcmatops, a contemporary 

 temnospondylous amphibian, as shown in the accompanying figure, after Williston 

 (fig. 35). The astragalus, it is seen, corresponds quite in its relations with the com- 

 bined tibiale, intermedium, and fourth centrale of the amphibian, with the perfo- 

 rating foramen at the junction of the intermedium and centrale with the fibulare, 

 precisely as in the carpus. The first and second centralia of the reptile likewise cor- 

 respond with the first two centraHa of the amphibian, each supporting its respective 

 tarsale. The third centrale plus fourth tarsale of the amphibian tarsus also occupies 

 the place of the enlarged fourth tarsale in the reptile. 



Note by Williston. The tibia in figure 35 is shown precisely as it lies in the matrix 

 in articulation with femur and tarsus. In the original figure the fibular side of the tibia 

 is shown, in the belief that the bone had suffered rotation. Such rotation, however, did 



Fig. 35. Trematops milleri Will- 

 iston. Left hind leg, ven- 

 tral side. Reduced. 



