BY R. BROOM. 551 



The question now arises whether the independent element in, 

 say, 07'7iithorhynchu>i, is homologous with the palatine process in 

 the majority of mammals. Unquestionably both occupy corre- 

 sponding morphological tracts, and by the large majority of 

 anatomists they have thus been regarded as homologous, and 

 such, until recently, was my own opinion. 



The skeleton in mammals differs from that of their Theriodont 

 ancestors mainly in the reduction of the number of skeletal 

 elements. In very many instances we find a certain region in 

 the Theriodont occupied by two elements, and in the mammal 

 the corresponding morphological region occupied by only one. 

 Where this occurs we have in most cases direct evidence that 

 what has happened is that one of the two elements has become 

 steadily reduced in size, and ultimately completely replaced by 

 the larger element, and not that the single element represents a 

 fusion of the original two. And we have further evidence that 

 the element which replaces the other has the form of the newly 

 developed part determined by the element which it replaces. 



Now, with regard to the palatine process of the premaxillary, 

 it seems much more satisfactory to regard it as a true portion of 

 the premaxillary which has replaced an anterior vomerine element, 

 such as the dumbbell bone of Ornitliorliynchus, than to regard it 

 as itself the homologue of that bone. We know that in the 

 higher Theriodonts (H) {e.g., Gomij^liognaihus) the tract occupied 

 by the palatine process in mammals is occupied by a pair of 

 apparently distinct small bones. The dumbbell bone of Ornitho- 

 rhynchus and the little median bone of MiniojyteA-ics represent 

 these bones of Gomi)hognaihiis anchylosed together. If Parker's 

 researches are confirmed we shall have in some of the Edentates 

 at least the condition where a rudimentary anterior vomerine 

 element is being replaced by a palatine process from the pre- 

 maxillary, and to which in the adult it is anchylosed. In the 

 large majority of higher mammals the anterior vomer is completely 

 aborted, and its place and functions taken by the replacing 

 palatine process of the premaxillary. 



