622 PROF. H. G. SEELEY ON [Aug. I9OO, 



II. Description of the Fossil. 



The Skull. 



Prof. Wiedersheim states that the surface of the skull was 

 smooth and without sculpture. The cast from the interior of the 

 skull gives practically no evidence of the extent of the cranial bones, 

 individually. The skull-bones which roofed over the head were 

 unusually thick, especially between the orbits of the eyes, where the 

 frontal bone is quite crocodilian in its thickness, flattened inferior 

 surface, and the way in which the orbits excavate its sides concavely. 

 These lateral curvatures are the chief means of determining the 

 positions of the eves : they appear to have been placed in the 

 middle length of the upper lateral margin of the head. 



In general form the skull is triangular, about intermediate between 

 Procolophon and Pareiasaurus. Apparently the head was closed 

 behind, more on the pattern of Rhopalodon, than of South African 

 genera. There is no descending pedicel for the lower jaw. 

 Prof. Wiedersheim figured indications of ten teeth in the pre- 

 maxillary and maxillary bones, which are represented as having 

 nearly cylindrical crowns on the right side ; but without any 

 indication of separation of incisors from molars, by a functionally 

 developed canine. In this respect, Aristodesmus parallels Procolo- 

 phon and Pareiasaurus. The snout is flattened, and transversely 

 rounded in front. The nares were manifestly terminal, though they 

 are not preserved. 



The internal mould of the skull in Procolophon makes a close 

 parallel to this fossil in the flattened, rounded, pre-orbital nasal 

 region, which is traversed by similar longitudinal ridges beneath 

 the nasal bones. The sides of the skull contract in front of the 

 orbits, in a way that is characteristic of Procolophon and of many 

 Theriodonts. 



On the palate there are obscure appearances, as of broken crowns 

 of short lateral rows of palatal teeth, like those of Procolophon. 



The Mandible and Teeth. (Figs. 1 & 2, p. 623.) 



The lower jaw is very short as compared with the length of 

 the skull, resembling in this respect both Pareiasaurus and Pro- 

 colophon. The relation in length between the lower jaw and the 

 skull in Pareiasaurus is as 11: 16, in Procolophon as 18 : 20, 

 and in this fossil as 12 : 16, showing very similar proportions. In 

 the inferior slab, the under aspect of the jaw is exposed. It has a 

 broad V-shaped form, which rather recalls Procolophon than Pareia- 

 saurus. . The median symphysis is as well-marked on the inner side 

 and base as in either of those genera. The splenial bone appears to 

 form the inner side of the jaw, which is concave from above down- 

 ward and in length, The inferior suture, which marks its junction 

 with the dentary bone, appears to run along the inferior edge, as in 

 Procolophon. As exposed, the inner side of the jaw does not increase 



