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ON SOME LITTLE-KNOWN BONES OF THE MAM- 

 MALIAN SKULL. 



By E. Broom, M.D. 



(Bead June 27, 1906.) 



The structure of the mammalian skull is usually assumed to be 

 pretty thoroughly known, and in most text-books descriptions or 

 diagrams are given of the various bones. Flower's well-known 

 diagram has been repeated with or without slight modifications in a 

 number of books, and many students have doubtless come to regard 

 the mammalian skull as a very simple structure compared with the 

 skull of the reptile. It may perhaps be in part owing to this 

 supposed simplicity that so many have rejected the idea of the 

 mammal being descended from a reptilian ancestor. In the present 

 paper, while I do not intend to discuss the question of the origin of 

 mammals, I wish to show that most of the cranial bones which are 

 supposed to be characteristic of the reptilian skull and absent in the 

 mammal can really be found in one or other of the representatives 

 of the mammalia. As there is to my mind no doubt that mammals 

 are descended from some Therapsidan reptile, most probably a small 

 Cynodont, I shall merely speak of the bones which are known to 

 •occur in the Therapsida, and which are not generally recognised 

 in the mammalian skull. 



Septomaxillary. 



The septomaxillary bone was first discovered by Kitchen Parker 

 in the skull of the lizard and snake, where it is of very large size. 

 In 1900 it was found by Howes and Swinnerton in the skull of 

 Sphenodon, and since then I have found it to be probably invariably 

 present in Therocephalians and probably also in all Cynodonts 

 though always absent in the allied Anomodonts. In the lizards 

 and snakes the bone is specialised to serve as a roof and protection 

 to the enormous organ of Jacobson, but in Sphenodon we find it in 



