146 Annals of the SoiUh African Musemu. 



palate, internal to the nasal openings, lie a pair of large prevomers. 

 These are undoubtedly the homologues of the very similar bones in 

 the front of the palate of Procoloj^hon, and, as the palate in Pareia- 

 saurus is essentially similar to that in Procolophon, also of the 

 paired elements in Pareiasaurus. In Tajnnocephalus , and in Titano- 

 suchus as we now know, there is also a pair of large bones in the 

 anterior palatal region, and there can be no doubt that these bones, 

 though no longer flattened out as in the earlier types, are the true 

 homologues of the Labyrinthodont pair. In the primitive Therio- 

 donts of the Lycosuchus type I have evidence to show, which will 

 appear later, that there is also a fairly well-developed pair of pre- 

 vomers. On reaching the higher Theriodonts, such as Gomplio- 

 gnathus, we find, as I pointed out in 1897," a distinct pair of small 

 prevomers, and these are precisely in the position of the two splints 

 which go to form the dumb-bell bone in Ornithorhynchus. 



It is thus manifest that the prevomer of Ornithorhynchus is the 

 homologue of the prevomers of the Labyrinthodont. 



The evolution of the vomer has not yet been so clearly made out. 

 We know, however, that there is a well-developed and very mammal- 

 like true vomer in Gompliognathus and some other higher Therio- 

 donts. We also find a true vomer in the Dicynodonts, and as these 

 are descendants of the earlier Theriodonts, doubtless a true vomer 

 will be found in the primitive Theriodonts. Nothing is known of the 

 posterior palatal region of Tapinocephalus or Titanosuchus, nor of 

 the deeper parts of the palate in Pareiasaiirus, but a distinct though 

 small true vomer occurs in Procolophon, which is doubtless the 

 homologue of the so-called " parasphenoid " in the Labyrinthodonts. 



* R. Broom, "On the Occurrence of an Apparently Distinct Prevomer in Gom- 

 phognathus," Journ. Anat. & Phys., Jan., 1897. 



