78 Prof. Owen on the Reptilian Fossils of South Africa. 



stone, when the Rhijnchosaurus manifested the Lacertian type of skull combined with 

 edentulous jaws, which most probably were sheathed with horn. But the Lacertian 

 type is more closely adhered to in the Rhynchosaur than in the Dicynodon : as, 

 for example, in the backward production from the upper angle of the orbit of the 

 column or arch of bone, formed by the post-frontal and mastoid running parallel 

 wdth the true zygoma, which is continued from the lower border of the orbit, as in 

 the skull of the Rhynchocephalus (PI. VI. fig. 5.). For other instances of the La- 

 certian affinities of the Rhynchosaur, I must refer to the Memoir descriptive of that 

 extinct o-enus*. It does not detract from the Lacertian character of the Rhyncho- 

 saur, that the slender curved and produced intermaxillary bones are double ; for 

 the median suture still remains in the analogously, but less extensively produced 

 intermaxillaries of the Rhynchocephalus above cited. What concerns us most in 

 the present inquiry is the anomalous edentulous sharp edge of the upper and lower 

 jaws in the ancient Rhynchosaur, and the Chelonian form of the deep lower jaw, 

 the same anomaly having been repeated in the extinct African Lizard of appa- 

 rently as remote a period, with the superaddition of Mammahan canine tusks. 

 For the rest, however, much difference of form is manifested in the two extinct 

 genera. Whilst the Rhynchosaur has the double, narrow and elongated inter- 

 maxillaries of the existing Rhynchocephalus, the Dicynodon presents the more 

 usual undivided condition of that bone in the existing Lacertians, but with a much 

 greater proportional development. 



It is interesting, however, to remark the same peculiar contraction of the cranial 

 cavity, indicating an arrested development of brain in both genera of ancient La- 

 certia. In the Rhynchosaur the temporal muscles met, and caused the develop- 

 ment of a median crest of bone along the whole of the parietal bone. Secondary 

 varieties which diversify the modern Lizards were also manifested by the two 

 most ancient forms of the order. The difference in the intermaxillary bone has 

 been already noticed. So with respect to the parietal bone : in the Rhynchosaur 

 it is imperforate, as in Cyclodus and Gecko ; whilst in the Dicynodon it is perforated, 

 as in Varanus. So likewise with respect to the mid-frontal bone : in the Rhyn- 

 chosaur it is single, as in Thorictes and Iguana ; whilst in the Dicynodon it is 

 divided, as in Lacerta proper and Varanus : again, each post-frontal bone is divided 

 by a suture in the Rhynchosaur, as in Iguana, but in the Dicynodon the post- 

 frontal is undivided, as in most existing Lacertians. The facial part of the skull is 

 as peculiarly compressed in the Rhynchosaur as it is expanded and abbreviated in 

 the Dicynodons. The dental peculiarity of the African Saurian forms its chief di- 

 stinction from the Rhynchosaurus, as from all other Sauria : but with the strange 

 superaddition of its two canine tusks, we must bear in mind that the affinities 

 * Cambridge Philosophical Transactions, vol. vii. part iii. p. 355. 



