PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 



SI 



lum, and a stout ectocondyle directed obliquely dorsad. The plane of the upper 

 extremity is relatively but little divergent from that of the lower. The rather 

 broad, spiral, and concave glenoid articular surface winds about the bone from the 

 dorsal outer side to the ventral inner side (fig. 31 a), and fits very perfectly into 

 the glenoid cavity of the scapula. As articulated with the scapula, the humerus 

 has a somewhat obliquely upward, outward, and backward position, showing con- 

 clusively that the animal in life could not have raised itself very high above the 

 ground. Indeed, the humerus could not have been brought much below a hori- 

 zontal position in life without partial dislocation from the scapular socket. The 

 lateral process is stout and is situated at about the junction of the upper and middle 

 thirds of the bone ; in the articulated position of the humerus it was directed down- 

 ward and a little forward. There is a small median process or eminence. The 

 ectepicondylar process is stout and protuberant, directed downward and forward 

 in the articulated position of the bone, and is situated a little beyond the middle 

 of the distance between the lateral process and the distal extremity (fig. 31 b). 

 The capitellum is unusually large, nearly circular in outline, and comprises about 

 one-third of a sphere ; the plane of its base is directed forward at an angle of about 

 45 degrees. The very stout ectocondyle is directed dorsad at an angle of almost 



Fig. 31. — Ophiacodon mirus Marsh. A, left humeras, ventral side, 

 X K; B, left humerus, distal end, X K; C, left ulna, radius, and 

 carpus, ventral side, X >i ; D, left carpus, dorsal side, X K- 



90 degrees with the plane of the entocondylar expansion ; 

 like the entocondyle, it has a terminal cartilaginous sur- 

 face for the attachment of powerful extensor muscles. This unusual dorsad inclina- 

 tion of the condyle is very characteristic of short, stout legs, and especially of short 

 humeri, as seen in Diadectes, Diasparactus, and Limnoscelis. The entocondyle is 

 very broad, and has an extensive surface on the distal border for the attachment of 

 strong flexor muscles. The articular surface for the ulna winds obliquely about the 

 distal border from the margin of the capitellum inward. 



Altogether the prominent and stout lateral processes, the protuberant ectepi- 

 condyle, probably for the attachment of the supinator muscle, and the dorsad 

 inclination of the ectocondyle indicate a powerful muscular control of the arm and 

 hand, very much greater than is seen in either Dimetrodon or Varanosaurus. 



Radius: The radius (fig. 31c), about three-fourths the length of the humerus, 

 has stout extremities and a relatively slender shaft. The truncated upper end is 

 subtriangular in outline, with its dorsal angle rounded and its articular surface 

 gently concave. The posterior surface above is flattened from side to side obliquely 

 inward; the dorsal surface is strongly convex. The somewhat obliquely truncate 



