Structure cmd Affinities of the Endothiodont Beptiles^ -275 



Opisthoctenodon agilis, Broom. 



This small Endothiodont which, like the previous one, was found 

 at Pearston, and in the same stratum, is represented by a fairly well 

 preserved skull, clavicle, interclavicle, sternum, humerus, radius, and 

 ulna of both sides, carpus and some phalanges of the right manus 

 and portions of the left. 



The skull is very like that of a small Oudenodon, and, apart from 

 the presence of molar teeth, I am not aware of any cranial character 

 that would distinguish the two genera. The skull has been already 

 figured in upper view (9). If this figure be compared with the 

 figures of Kistecephalus arctatus given by Owen, it will be seen that 

 the resemblance is sufficiently close to make it possible that Owen's 

 species may belong to Opisthoctenodon. Lydekker (4) expresses 

 doubt as to its being a Kistecephalus. One of the specimens (47088) 

 at least most probably belongs to this genus. 



The teeth are confined to the maxillary and dentary bones. There 

 appear to be five mature teeth, and there are very clear evidences of 

 replacing teeth developing to the inner sides of the mature ones. 

 Those teeth which are well preserved are seen to have a smooth 

 anterior edge and a deeply serrated posterior border, there being 

 seven denticulations on the only perfect tooth. 



The lower jaw is unusually flat and broad in front. 

 The clavicle is long and slender and very mammal-like. It is 

 curved almost exactly as in man. 



The interclavicle is a small, flat, shield-like plate, of which the 

 anterior border is moderately straight and the posterior border 

 rounded. 



The sternum is an irregular, rounded flat plate, which is consider- 

 ably broader in front than behind. It differs greatly in appearance 

 from the interclavicle in that while the latter is formed of dense 

 bone, the sternum has been either cartilage or ossified cartilage or 

 very spongy bone. 



The humerus is typically Dicynodont. The large deltoid ridge. 

 extends over more than half of the bone. At its lower margin is 

 situated the entepicondylar foramen. 



The radius and ulna are not very well preserved, but both bones 

 are fairly similar to those of Oudenodon. The ulna has the olecranon 

 process considerably less developed than in Oudenodon trigoniceps. 



The carpus I have already described elsewhere. It consists as in 

 Oudenodon, Theriodesmus, and most primitive reptiles, of four 

 elements in the proximal row, viz., radiale, intermedium, ulnare, 

 and pisiform, of two centralia, and of a distal row of carpalia. In 



