BY J. T. WILSON. 141 



ginous nasal septum. The latter, however, is clad for a 

 certain distance forward on its ventral border by a bilaterally 

 symmetrical anterior vomerine splint, distinct from, but in series 

 with, the principal vomer ; while in front of this the ventral 

 aspect of the septum is bare — covered only by the mucous mem- 

 brane of the mouth — and here it descends, flattens dorso-ventrally, 

 and spreads out into a " prenasal plate " of cartilage, which is 

 continuous laterally and behind with the aliseptal cartilages, and 

 in front with the marginal cartilage of the upper "lip." [See 

 paper (1) and figures in Macleay Memorial Volume.] 



Further, with regard to the internasal aperture, I regard it as 

 situated in a position quite definitely deterniined by the course of 

 development, viz., above the extreme anterior end of the secondary 

 palate. It seems less difficult to understand non-coalescence of the 

 septum with the secondary palate either posteriorly or anteriorly 

 than it would be to imagine an arbitrary interruption of the 

 coalescence midway. Posteriorly a persistent median choanal 

 passage would be the result of premature cessation of the process 

 of coalescence, while non-union anteriorly such as we have in the 

 case befoie us may possibly be explained by supposing that the 

 porterior boundary of the internasal aperture really represents 

 the original starting point of palatal coalescence, and that the 

 extension of the secondary palate in front of that point, at a lower 

 level than the septum, is the product of a later development. Of 

 course the point can only be decided by actual embryological 

 investigation, for which, unfortunately, the material has hitherto 

 not been available. 



The anterior boundary of the fenestra is in my view due to a 

 rapid ventral descent of the septum into the roof of the mouth in 

 front of the anterior border of the permanent palate, and I 

 conjecture that it is probably formed somewhat late in develop- 

 ment, as the septum undergoes the great anterior elongation 

 which it acquires in the snout of this long-nosed animal. 



It might detract from the value of the arguments based upon 

 the posterior relations of the dumb-bell bone to the internasal 

 aperture if it could be contended that the latter may be merely 



