52 PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 



distal end is broadly subtriangular, with the dorsal angle rounded, the inner angle 

 more acute. The two sides of the bone are nearly symmetrically concave in outline. 



Ulna: The ulna (fig. 31 c) is a longer and stouter bone than the radius, and 

 is almost or quite the length of the himierus. The olecranon is considerably pro- 

 duced, and has its distal cartilaginous surface continuous with that of the sig- 

 moid fossa. Here, again, it is evident that the triceps was unusually powerful and 

 that its action was exerted at much mechanical advantage. The inner border of 

 the bone is deeply concave and is thicker than the outer or radial border. The 

 distal extremity, somewhat less expanded than that of the radius, has its truncated 

 articular surface for union with the carpus oblique to the long axis of the bone. 

 Its inner angle is convex, evidently for articulation with a pisiform bone, which, 

 however, has not been found. The ventral side of the bone is gently concave 

 longitudinally above, convex distally. 



Carpus: The carpus was found in almost perfect articulation on the right side; 

 the bones forming it on the left side were slightly displaced. On each side the 

 carpus has been restored so perfectly to its original condition that scarcely anything 

 more could be desired. The number and arrangement of the bones are quite as in 

 Dimetrodon and also as in Varanosaurus, except that the first centrale and the fifth 

 carpale are well ossified in Ophiacodon. The radiale is a short, thick bone, very 

 convex in front. The intermedium is elongate and helps to form a half of the 

 articular surface for the ulna. The ulnare is also elongate and rather narrow; its 

 inner margin curves ventrad. The perforating foramen is rather large, formed 

 by the ulnare, intermedium, and second centrale. Of the five carpalia the fourtli 

 is the largest, as usual, the third is next in size, the fifth is small, and the second 

 is not much larger. The first centrale is intercalated between the radiale, second 

 centrale, and first three carpalia. The second centrale is larger, articulating with 

 all the bones of the carpus except the first and fifth carpalia. The carpus, as 

 articulated, is convex from side to side on the dorsal side, concave on the palmar. 

 The articulations of the radiale, intermedium, ulnare and second centrale are all 

 close, permitting little motion between them. The articulations of the first centrale 

 and all the carpalia are loose, not only with each other, but also with the bones of 

 the first row, especially so on the radial side of the wrist; the free surface of the 

 first centrale is broader behind than in front. 



Hand: The bones of the right hand were all found in intimate articulation 

 with each other and with the carpus, save the distal two phalanges of the thumb, 

 which doubtless were lost in collection. Of the left hand the distal phalanges of 

 several fingers could not be found, but the proximal phalange of the thumb was in 

 position, giving the complete structure of the hand, with the exception of the ungual 

 phalange of the first digit. The thumb metacarpal is, as usual, small and broad; 

 the second, third, and fourth metacarpals increase markedly in length, but not 

 nearly so much so as do the corresponding ones of Varanosaurus. The fifth meta- 

 carpal is about two-thirds the length of the fourth ; it is slender and curved, and was 

 evidently divaricable in life; possibly, as in many lizards, it was freely separable 

 in life to the base of the metacarpal. The ungual phalanges differ markedly from 

 those of other known American Pelycosauria or Theromorpha in their less curved 

 and less pointed form. They are more like nails than claws, almost flat on the 

 under side, somewhat thinned at the extremity, but not at all pointed. The animal 

 could have made no use of them in climbing, or as offensive or defensive weapons. 



In figure 30 the right arm and hand are shown almost precisely in the position in 

 which they were found in the matrix, save that some of the digits have been straight- 

 ened out and a few of the loosened carpal bones have been more closely adjusted. A 



