1845.] 501 



striking and peculiar as to require a detailed osteological descrip- 

 tion for their complete illustration. 



In tliese animals, the Crocodilian sti'ucture is chiefly manifested 

 in the occipital region of the skull, and gives place to the Lacer- 

 tian characters in the upper and fore part ; but in regard to these 

 deviations it must he remembered, that the distinctive features of 

 the Crocodilian type are most broadly manifested in the existing 

 representatives of the order, and are modified and rendered less 

 salient in the more numerous and varied extinct members. 



It is necessary to bear in mind this tendency to the amalgama- 

 tion of Crocodilian and Lacertian characters in the older Loricata, 

 in order to foi'm a right estimate of the value of those cor- 

 respondences with the cranial peculiarities of the existing La- 

 certians. 



Nevertheless, various characters justify the conclusion, that the 

 general type of cranial organisation manifested by modern lizards 

 was that in which the peculiar modifications of the Dicynodon have 

 been superinduced. It is not, however, amongst the modern 

 lizards that we find the nearest approximation to the Dicynodon. 

 For this we must go as far back into the period of Reptilian life 

 on this planet as the epoch of the new red sandstone, when the 

 Rhynchosaurus manifested the Lacertian type of skull, combined 

 with toothless jaws, which were most probably sheathed Avith 

 horn. 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 apparently as remote a period, with the super- 

 addition of Mammalian canine tusks. For the rest, much dif- 

 ference of form is manifested in the two extinct genera ; but it 

 is interesting to remark the same peculiar contraction of the 

 cranial cavity, indicating an arrested dev elopement of brain in 

 both of them. The dental peculiarity of the African Saurian forms 

 its chief distinction 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 linking the Dicy- 

 nodon to Crocodilians and Chelonians are much more strongly 

 manifested than they are in the Rhynchosaurus. 



The author, in concluding his account of the Dicynodon, ad- 

 verts to the analogy of structure, which radiates from this genus 

 in the direction of the Ophidian division of existing Reptilia, 

 although it is unsupported by any other concordances of 

 cranial or dental organisation than those about to be cited. 

 In the poisonous serpents, the rattle-snakes for example, the 

 intermaxillary bone is single and edentulous ; the maxillary 

 bone supports a long, curved, pointed tooth, which, when ad- 

 vanced, descends outside the lower jaw. Apart from all the 

 other peculiarities of the maxillary and dental systems of the 

 poison-snakes, they alone, of all existing Reptilia, repeat, in the 

 above-cited structures, the characters of the Dicynodon. But, in ad- 



