Yol. 56.] ARISTODESMUS RUTIMEYERI. G33 



rounded distal surface to the humerus as in Rhopalodon and allied 

 types, while in this fossil the resemblance in the mode of articulation 

 of the radius is closer with Monotremes. Several types of humerus 

 are included under the Anomodontia. In Pareiasaurus the distal 

 expansion of the hone may be so far lost, that it extends only a 

 short distance on each side of the trochlear articular surface. The 

 expansion of the proximal end, the constriction of the shaft, and the 

 reflexion of the radial crest are closely paralleled in Procoloj>hon. The 

 variability of the humerus in Theriodonts such as Herpetoclieirus and 

 Theromus leads to the conclusion that many of the resemblances of 

 Aristodesmus to Echidna are adaptive modifications ; that the affinity 

 of Aristodesmus to the Anomodonts is much closer than to Echidna: 

 and that Aristodesmus approaches more closely to Echidna than any 

 Anomodont hitherto known. 



(b) The Ulna and Radius. 



The ulna and radius are unequal strong bones exposed side by 

 side. Their proximal ends are partly hidden beneath the distal end 

 of the humerus, and they are not easily determined, because there 

 is some appearance in one impression of the slender bone extending 

 beyond the transverse process of the distal end of the humerus, so 

 that the ulna is anterior to the radius, as in Ornithorhynchus. 



The radius is a flat strong bone, its transverse width greatly 

 exceeding that of the ulna. The proximal end is Vvide, and trans- 

 versely truncated. The distal end is a little narrower, and excep- 

 tionally convex from, within outward. There is some appearance 

 of a contact-surface upon the radial border of the distal end, but no 

 corresponding surface is seen upon the ulna. The inner side of the 

 radius is markedly concave. The external borders of these bones 

 are relatively almost straight. A shallow vertical channel begins 

 in its lower third, and increases in depth proximally. This makes 

 a prominent ridge at the proximal external border, which shows 

 ligamentous roughnesses ; and on the inner side a ridge is strongly 

 developed, thickening the bone, which widens the radius so as to 

 come into contact with the ulna at its proximal end. 



The stoutness of the radius is a divergence from some African 

 Anomodonts, in which the condition of the bone is at present 

 imperfectly known. There is some evidence that the proximal end 

 of the ulna extended appreciably beyond the radius in the manner 

 of Herpetoclieirus. 



The ulna is more expanded and massive at the distal than at 

 the proximal end ; it has the shaft somewhat flattened behind, 

 and narrowed in front by side-to-side compression. The distal 

 articulation is a truncated transversely-ovate flat surface wider than 

 deep, which has a sharp margin, and is inclined somewhat obliquely. 

 There is no single carpal bone exposed which would fit the distal 

 extremity of the ulna. The sides of the ulna are concave from 

 above downward, especially the inner border, its external side being 



