BY J. T WILSON. 139 



(2) That this peculiar relation of the postaxial end of the 

 dumb-bell to the cartilage of the nasal floor is no trivial or insig- 

 niticant fact, is confirmed by the prolongation backwards of the 

 vertical part of the bone some considerable distance dorsad of the 

 maxillary palate and in relation to the cartilaginous septum. It 

 appears to me that a bone which is so prolonged backwards on a 

 higher plane than the maxillary palate cannot be regarded as 

 developed in the same morphological plane with it, even though 

 anteriorly it has come down so as to occupy the same actual plane, 



(3) But further, we have seen not only that the dumb-bell bone 

 is prolonged backwards in the form of a bifurcated (vomerine) 

 splint in relation to the ventral edge of the cartilaginous septum 

 nasi, altogether above the plane of the maxillary palate, but also 

 that this vomerine spur is separated from the maxillary palate by 

 a very peculiar hiatus. In what light are we to regard the inter- 

 nasal passage above referred to ? A very little consideration will, 

 I believe, suffice to render this somewhat extraordinary feature 

 of an adult mammalian septum nasi quite intelligible. When 

 the palatal plates of the embryonic maxillary processes coalesce to 

 form the floor of the nasal cavity, they very soon unite with the 

 ventral edge of the internasal septum. This coalescence generally 

 proceeds backwards towards the posterior nares, and before the 

 coalescence of the palate with the septum is complete posteriorly 

 there is a single median choanal passage, i.e., the nasal cavities 

 freely communicate. It is plain that here in Ornithorhynchus we 

 have a condition of non-coalescence of the palate with a certain 

 extent of the ventral border of the septum nasi.* But it is 

 notable that this non-union does not occur towards the po.sterior 



* Attention may here be called to the instructive similarities in the 

 general relations of parts between transverse sections through the nasal 

 region of Platypus in the region of the internasal aperture, and similar 

 sections through the nasal region of many embryo mammals passing through 

 the embryonic choanal communication between the two cavities. Cf. in 

 particular, figures in Parker's monographs on development of Mammalian 

 skull, e.rj., Edentata and Insectivora, pi. iii. figs. 9 and 9a. Note especially 

 the ventral relations of the cartilaginous septum to the vomer, &c. 



