140 ON THE "dumb-bell-shaped" BONE IN ORNITHORHYNCHUS, 



nares, but in front of the vomer. The question arises, why should 

 the failure to unite with the septum have taken place precisely 

 in this region? This is an important question, and I think it 

 may easily be answered if we recognise that the anterior end of 

 the osseous maxillary palate indicates the anterior limit of fusion 

 of the palatal plates of the secondary or permanent palate. In 

 other words the secondary palate ceases somewhat abruptly with 

 the anterior margin of the maxillaries, and in front of this the 

 floor of the nasal cavities is constituted, not by a secondary palatal 

 formacion at all, but simply by the ventral parts of the cartilagi- 

 nous walls of the primary nasal capsules which are intimately 

 bound up with the forward extension of the intertrabecular carti- 

 lage forming the cartilaginous septum nasi. This, indeed, is 

 demonstrably the case. The wide area between the diverging 

 premaxillo-maxillary crura in the macerated skull is largely filled 

 up in the recent state by a sheet of cartilage whose composition 

 has been described and figured in the paper already referred to 

 (1), and wiiich has nothing to do with the secondary palate. In 

 ordinary adult mammals the area, which is homologous with this 

 interval, is closed in below the nasal cavities by the premaxillaries, 

 which send inwards and backwards palatine plates* which join 

 the maxillary palate, completing the secondary palate in front. 

 And according to the prevalent theory the dumb-bell bone in 

 Ornithorhynchus represents these premaxillary palatine plates, at 

 least in part. I am of opinion, however, that the facts I have 

 adduced respecting the dorsal and posterior relations of the dumb- 

 bell bone tend to negative the view that that bone is an ossifica- 

 tion in the morphological plane of the secondary palate, and point 

 distinctly to its homology to a bone of the vomerine series. To 

 sum up this portion of the argument : I regard the secondary 

 palate as ceasing altogether at the anterior margin of the osseous 

 maxillary palatine plate. The failure to develop in front, on the 

 part of the premaxillary moiety of the usual secondary palate, 

 results in the exposure from below of the ventral walls of 

 the cartilaginous nasal capsules, and of the intervening cartila- 

 * See, however, discussion towards the end of this paper. 



