166 Prof. H. G. Seelej on the 



of teeth upon the palate. There is no evidence of the skele- 

 ton associated with either skull ; but the skull in the latter 

 type is of some interest from its excellent preservation, which 

 shows the sutures. In a previous paper (Phil. Trans. Roj. 

 Soc. 1889j B, p. 290) I have drawn attention to a certain 

 parallelism between tlie bones which cover the skull and the 

 elements which roof over the spinal cord in some cartilaginous 

 fishes. This now appears to me not only to explain why the 

 median bones of the skull are sometimes single and sometimes 

 paired, but to elucidate the presence of three bones in the 

 Anomodontnow to be described and in some other Vertebrata. 

 Normally every single median bone, such as the inter- 

 maxillary (which alone is recognized in the Bidentalia termed 

 Dicynodonts), is flanked by a pair of lateral bones — the pre- 

 maxillaries. When the one is developed, the other commonly 

 loses its individuality. So that the Theriodonts have a 

 moderate development of premaxillary bones, but in Dicyno- 

 donts the intermaxillary is as strongly developed as are the 

 premaxillary bones in Icldhyosaurus. The bone which 1 

 have termed infra-nasal in Dicytiodon may be the premaxillary. 

 The second median ossification in the skull — the ethmoid of 

 birds — does not reach the surface in Dicynodonts, but appears 

 to be related to the paired nasal bones in a similar way, though 

 all three are rarely developed together on the surface of the 

 skull. Next succeeds the single frontal of lizards with the pair 

 of frontal bones on its flanks, followed by the single parietal 

 and the pair of parietal bones ; the last being the interparietal 

 and the pair of superoccipital bones of Labyrinthodonts. 

 Thus the roof of the skull would include the equivalents of 

 five vertebral arches of fishes if all the elements were simul- 

 taneously developed. Without such a recognition of homology 

 the presence of such bones as the interparietal, preparietal *, 

 interfrontal, and intermaxillary cannot be explained. Tiiese 

 median superior ossifications of the skull characterize Dicyno- 

 donts more than any other group of Anomodonts. 



I have seen but one imperfect skull of Mochlorhinus platy- 

 ceps. It was found many years since at Bethulie, a little 

 north of the Orange River, near Aliwal North, by Mr. J. G. 

 Donovan, who presented it to the Albany J^luseum. There I 

 examined it in 1889. I am indebted to Dr. W. G. Ather- 

 stone, F.G.S., and the trustees of that museum for the oppor- 

 tunity of studying the specimen in this country and of 



* The bone here named preparietal is named interparietal by Mr. E. T • 

 Nowton in Phil. Trans. B. 8(5, 1893, in explanations of plates xxvi. and 

 xxxii. 



