1901.] AFFINITIES or riDENODOJS'. 181 



very little doubt that the Platypodosaihrus robustus remains are the 

 bones of a large species of Udenodon. The large skull figured by 

 Owen (3) as Udenodon magnus bears almost the same proportion to 

 the limb-bones of " Platypodosaurus robustus " that the skall of 

 Udenodon gracilis does to its Kmb-bones. It seems, therefore, very 

 probable that the bones described as Platiipodosaurus robustus are 

 parts of the post-cranial skeleton of Udenodon magnus, Owen. 

 Lydekker (7), in his British Museum Catalogue, holds that 

 Udenodon magnus is a synonym of Udenodon jyrognatlms Owen. 

 It is impossible for one away from the original specimens to offer 

 an opinion on the subject, and though the figures seem to show 

 a number of distinctive characters, Lydekker's work among the 

 Anomodonts has been so carefully and conscientiously done, that 

 any opinion expressed by him must always cany very great 

 weight. 



Affinities of Udenodon. 



The genus most nearly allied to Udenodon is Dicynodon ; in 

 fact so great is the resemblance between the two genera that it 

 has been suggested that Udenodon was the female of Dicynodon. 

 The discovery of the limb-bones shows that the two genera are 

 distinct though very closely related ; and there seems to be very 

 little doubt that Udenodon is merely a slightly modified Dicynodon 

 in which tlie tusks have ceased to develop. 



When the bones of Udenodon or Dicynodon are compared with 

 those of other Vertebrates, we find that the group has many marked 

 affinities with the Theriodonts and the Mammals ; some affinities, 

 but less marked, with the primitive reptilian types such as Paria- 

 saurus, and even with the E;hynchocephalians, Plesiosaurs, and 

 Chelonians ; but that the relationships with the more highly 

 specialized reptiles, such as the Crocodiles, Dinosaurs, and Ptero- 

 dactyles, are rather I'emote. 



In Udenodon the anterior half of the skull is so greatly modified 

 in connection with the toothless beak that the affinities are 

 masked. There is little doubt, however, that the beak is derived 

 from that of a Theriodont type, hitermediate stages being found in 

 Cryptocynodon and Dicynodon. 



In the structure of the posterior part of the skull Udenodon 

 agrees closely with the Theriodonts and Mammals ; and differs 

 markedly from all the regular Reptilian types. 



In the majority of typical reptiles the temporal region of the 

 skull is protected by two bony arches — a supra-lateral arch formed 

 by the postf rental or postorbital and the squamosal or supra- 

 temporal, and an infra-lateral formed by the jugal and quadrato- 

 jugal. This arrangement, or a modification of it, occurs in the 

 Ehynchocephalians, Pelycosaurians, Crocodilians, Dinosaurs, and 

 other groups. The more primitive lizards differ in having lost 

 the lower arch. In the Dicynodonts, as in the Theriodonts and 

 Mammals, there is but a single arch formed by the jugal and 

 squamosal ; and some difference of opinion has been held as to 



