142 ON THE " DUiMB-BELL-SHAPED " BONE IN ORNITHORHYNCHUS, 



an adaptive character in this peculiar animal, and one which may 

 not bear all the significance which I attach to it. But this 

 position can hardly be taken up when I am able to state that the 

 feature appears to be a general character of the Monotremata since 

 it is also present in the genus Echidna (see fig. 5). I have not 

 yet had time fully to investigate the structural relations of the 

 fenestia in the latter type, though the material is in process of 

 preparation. I fully expect that a study of the same region in 

 Echidna along parallel lines will throw a fuller light on the whole 

 anatomy of the region, and will, perhaps, enable us to determine 

 what in that animal is the structural homologue to the dumb-bell 

 bone in Ornithorhynchus. 



After diligent search I have been unable anywhere to find a 

 reference to this very obvious perforation of the wall between the 

 nasal chambers in Ornithorhynchus and Echidna, or to the 

 persistence of such an aperture in any other mammalian form. I 

 do not think that its presence can ever have been recorded, else 

 its significance would hardly have been overlooked by comparative 

 anatomists. 



The aperture does, however, bear a most interesting resem- 

 blance to that which in the duck and certain other water birds 

 (15) perforates the septum opposite the external nostrils. This 

 also is a low and anteriorly elongated aperture in the septum at 

 the nasal floor, and the chief superficial difierence from the inter- 

 nasal aperture in the Monotremes lies in the more anterior position 

 of the aperture in the duck's nose. 



It may yet be contended that the considerations which I have 

 hitherto brought forward are, after all, insufiicient to enable us 

 finally to dispose of the argument in favour of the homology, — 

 derived from a comparison between their relations to the organ 

 of Jacobson and the naso-palatine foramen, — between the dumb- 

 bell bone in Ornithoi'hyachus on the one hand, and the palatine 

 plate of the ordinary mammalian premaxilla on the other, or 

 rather that portion of the latter which, in the words of Sir 

 "William Turner, " lies between the incisive foramen and the 

 mesial palatal suture." 



