726 SNOUT OF FCETUS OF MONOTREMES, 



It is now unnecessary to enter into any detailed criticism of 

 Symington's objections to the theory of the vomerine nature of 

 the dumb-bell bone, as the jDresent writer is of opinion that the 

 definite prevomerine homology of at least the greater part, if not 

 of the whole, of the palatine process of the mammalian premaxilla, 

 may now be taken as sufficiently established, and this position 

 practically conserves that view of the homology of the bone for 

 which Symington contended. Indeed in a letter to the writer 

 Professor Symington says, with reference to the dumb-bell bone, 

 thar he "does not think there is now much difference of opinion 

 between us as to its homology." 



The question of whether the entire palatine process of the 

 premaxilla in mammals is prevomerine, as Broom is disposed to 

 believe, cannot yet be regarded as finally decided. Reason will 

 now be adduced for the belief that the anterior part of this 

 process may originate as a direct backward extension from the 

 body of the premaxilla, whilst the posterior part arises as a 

 distinct prevomerine element. Such a view is very strongly 

 supported by W. K. Parker's investigations on various Eutheria. 

 The figures illustrative of the present paper prove conclusively 

 that it holds good for Ornithorhynchus, provided we accept the 

 homology of the dumb-bell bone to any portion of the palatine 

 process of the premaxilla in other mammals. 



In both specimens of the young of Ornithorhynchus, and also 

 in the . young Echidna the main body of the premaxilla is com- 

 pletely divided into dorsal and ventral plates grafted as splints 

 upon the dorsal and ventral surfaces of the wide-spreading cartila- 

 ginous place which forms the chief skeletal element of the snout 

 extending into the lip. These dorsal and ventral parts of the 

 premaxilla are quite distinct from one another, as was surmised 

 to be the case in young stages, in the earlier paper on the snout, 

 contained in the Macleay Memorial Volume (6). 



The ventral plate of the body of the premaxilla appears from 

 below as an elongated and curved strip of bone applied to the 

 ventral surface of the cartilaginous plate aforesaid. As seen in 

 Model i., (fig. 3) it tapers away posteriorly, and is there lodged 



