682 



Dn. R. imOOM ON THK STRUCTURE OF THE SKUIiL 



met the upper pnit of the outer plate, tlie condition would be as 

 in TUanosiidms. The Dicynodont angulnr represents whivt was 

 probably the pre-Titanosnc])i:in condition. The inner view of 

 the jaw shows the large prearticnlar and the al)sence of the 

 cora(!oid. The whole inside of the jaw is seen to he strikingly 

 siujilar to that of TiUmosuchus. The Dicynodon jaw might 

 almost be looked upon as a. degenerate Titanosuchian jaw which 

 had lost the rudimentary coronoid, lost the teeth, and developed 

 a new type of articulation. 



The JiJndothiodon jaw is a primitive Anomodont type. It 

 diflers from that of Dicynodon in being a more loosely put 

 together jaw. The pi-earticular is more slender, and the splenial 

 considerably longer. There is, as in Dicynodon, no trace of a 

 coronoid. 



Text-ilffure 15. 



lUglit inaiiililile of JCmlofliiinlou (in<iiisliceps, sj). iiov. 'I'liis is \\w jaw of llie 

 Eiidotliioilon inoiinted in tlio Aineiiciiii IMiiseiiin xvliidi 1 liml leforreil to 

 E. nnisevies Owen. Jt cliffevs in tlio iivrniigcment of tlu' tt'cth and in tin.' 

 jaw being longer and narrovNor. 



The Titanosnclms jaw differs from that of the Pelycosaui's in 

 the fact that these have only a slight indication of the peculiar 

 development of the angular, and also that in these latter there is 

 a long well-developed coronoid. AVhen I first described the jaAV 

 oi Diinetrodon,! thouglit there were in the Washington specimen 

 two coi^onoid elements. Williston on re-examining the si;ecinien 

 considered that the supposed dividing suture is a crack. I have 

 had no chance of again examining the specimen, but it is rpiite 

 likely Williston is right on this point. 



Another jaw that may be compared with that of TUanosuc/uis 

 is the Plesiosaurian jaw. I do not consider the Plesiosaur.s 

 nearly lelated to the Therapsids. Some others do. But it must 

 be admitted that there is considerable resemblance in the jaw 

 structure. Peloneustes has a very large splenial, a long coronoid, 



