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



selves when we come to a closer comparison. Where the orbits look out from 

 the Tortoise's skull, the anterior nostrils open in that of the Dicynodon : where the 

 single nostril is perforated in front of the skull of the Tortoise, we find a broad, 

 convex, solid wall of bone in the Dicynodon, 



The resemblance to the Tortoise is due merely to modifications of proportion and 

 shape of the bones ; in the essential anatomical structure of the skull, the present 

 fossil agrees with the more complete skull of the Dicynodon lacerticeps : and seeing 

 in both the same aspect of the orbits, the same lateral position of the two distinct 

 nostrils, the same expansion of the maxillary bones with their singular dental 

 weapons, we may safely conclude, that the parts which are complete in the skull 

 of one species must have differed from those which are mutilated or wanting in 

 the other species, only in their form and proportion, and not by any essential anato- 

 mical modifications. With this conviction, the entire and well-preserved intermax- 

 illary and palatal region of the skull of the Dicynodon testudiceps gives important 

 additional evidence of the nature and affinities of the extinct genus. 



All the Crocodilia retain the median suture of the intermaxillaries ; most of the 

 Chelonia likewise retain it * : in neither order is there any median ascending inter- 

 maxillary process. The Ophidians and most of the Lacertians (all of them accord- 

 ing to Cuvier)t possess a single intermaxillary bone, which sends a median process 

 upwards to the interspace dividing the nostrils. In these orders the type of the 

 intermaxillary bone of the Bird is foreshadowed, but the size of the bone is very 

 diminutive as compared with Birds. 



In the portion of the skull of the Dicynodon testudiceps the whole extent and 

 structure of the os intermaxillare (19) is clearly displayed: there is no trace of 

 median suture on either the external or palatal portions of the bone : it has a me- 

 dian * branche montante,' which, at the middle emargination of its upper sinuous 

 border, receives the extremities of the nasal bones (1 5): as it descends from the inter- 

 nasal space it expands, and at the fore-part of the lower border of the nasal aperture 

 meets and joins the superior maxillary bone. The Dicynodon, by the superior size 

 of the intermaxillary bone, pushes the resemblance of its cranial organization to that 

 of Birds much further than do the other Reptilia with the same undivided condition 

 of that bone. I may here however remark, that some of the ancient Saurians, 

 more especially the marine forms, are remarkable for the greater proportion of their 

 divided intermaxillaries than is manifested by the modern Crocodilia. The inferior 

 trenchant border of the intermaxillary bone in the Dicynodon testudiceps describes 

 nearly a semicircle horizontally, between the sutures with the maxillary bones 

 (PI. V. fig. 1, 20). The middle part of the edentulous border is a little produced and 



* Chelys presents a very small, single, undivided intermaxillary, 

 t Le9ons d'Anat. Comparee, ed. 1837, tom. ii. p. 531. 



I 



