129 



OBSERVATIONS UPON THE ANATOMY AND RELA- 

 TIONS OF THE "DUiMB-BELL-SHAPED" BONE 

 IN ORNITHORHYNCHUS, WITH A NEW THEORY 

 OF ITS HOMOLOGY; AND UPON A HITHERTO 

 UNDESCRIBED CHARACTER OF THE NASAL 

 SEPTUM IN THE GENERA ORNITHORHYNCHUS 

 AND ECHIDNA. 



By J. T. Wilson, M.B., Professor of Anatomy in the 

 University of Sydney. 



(Plates VIII. -IX.) 



(A preliminary note .summarising most of the conclusions 

 arrived at in this paper was presented at the 'Meeting of the 

 Society March 28th, 1894, and published in the Abstract of 

 Proceedings of that date). 



The question of the morphological significance of the dumb- 

 bell-shaped bone in Ornithorhynchtos first attracted the attention 

 of the writer when investigating, with Dr. C. J. Martin, the 

 anatomy of the muzzle of this animal. In the paper in the 

 Macleay Memorial Volume (1) in which we recorded the results 

 of our work, no new opinion upon the subject of the present paper 

 was expressed, though even then our series of coronal sections 

 had gone far to convince me that the usual premaxillary theory of 

 the nature of the dumb-bell bone was an inadequate one. This 

 latter view was that entertained by Rudolphi and Meckel (2, p. 20) 

 and was adopted by Owen in his article on the Monotremes in 

 Todd's Cyclopaedia (3). 



But in his later work upon the Anatomy of Vertebrates 

 (4, p. 322) Owen apparently forsook this theory in favour of an 



