1901.] ArriNiTTES or UDENODOisr. 169 



Lydekker in the Britisb Museum Catalogue of Fossil Reptiles, 

 which is probably that ot a species of Udenodon. The supra- and 

 exoccipitals are bordered by the interparietal and the squamosals, 

 while the lower corners of the large exoccipital processes also 

 articulate with the quadrates. 



Between the quadrate and the descending process formed by 

 the exoccipital and basioccipital, there lies a remarkable little 

 dumbbell-shaped bone, which with one end fits into a hollow of 

 the occipital process and with the other supports the quadrate. 

 As it is but loosely articulated, it is lost from the majority of 

 Dicynodon and Udenodon skulls discovered. This bone differs so 

 markedly from any bone found in the posterior region of the skull 

 in known Eeptiles or Mammals, that one hesitates in giving an 

 interpretation. As, however, it forms with the notch in the 

 lower border of the exoccipital an oval aperture, and as the 

 columella auris lies in this same notch of the exoccipital, it seems 

 to me most probable that it is the homologue of the mammalian 

 tympanic. 



Lower Jaw. 



The lower jaw is almost typically reptilian in structure {cf. text- 

 fig. 10, p. 165). In front, the two large toothless dentaries (d.) 

 are anchylosed together as in the tortoise. Each dentai'y is con- 

 siderably deeper than in the tortoise, and differs in forming a 

 single edge above, instead of two ridges as in the Chelonian. In 

 U. yreyi the outer surface of the dentary is moderately flat ; but 

 in U. gracilis there passes outwards from the posterior part of 

 the bone a very prominent horizontal ridge. On passing back- 

 wards the dentary divides into an upper and a lower lobe, which 

 meeting respectively the suraugular {sa.) and the angular (a.) 

 encloses with these a fair-sized oval vacuity. The angular 

 is a rather large flat element which articulates with the dentary 

 in front, the surangular above, the splenial below, and the articular 

 (ar.) behind. The surangular is a fairly strong bone which fits 

 into a deep cavity in the posterior end of the upper part of the 

 dentary. The splenial extends along almost the whole length of 

 the jaw, from the articular behind to the symphysis in front. 

 Posteriorly it is fairly stout, but on passing forwards it becomes 

 a rather thin plate. The articular is a large thick bone, but, as in 

 Ohelonians, short. 



I am unable to give any account of the hyoid apparatus, as 

 though there are evidences of hyoid bones, they are disconnected 

 and their interpretation is quite uncertain. 



Vertehroe. 



In the little skeleton of Udenodon gracilis (PL XVI.) most of 

 the vertebrae are preserved, but none are well displayed. The atlas 

 and axis are hidden by matrix, but from the 3rd cervical (4th ?) 

 there are indications of almost all the other vertebrae. The skeleton 

 does not show where the division lies between the cervical and 



