BY J. T. WILSON. 133 



breadth of the bone [in the posterior segment] is due to the 

 elongation of the horns of the crescents which reach about half- 

 way round the organ of Jacobson and are in close contact with 

 its cartilage." This author does not mention the vomer, but quite 

 plainly he does not adopt the view that the vomer is fused with 

 the dumb-bell dorsally. 



He concludes by stating that " the dumb-bell bone from its 

 position in relation to the cartilages of the nose is evidently 

 ossified in the membrane investing them," and that " the relation 

 of the bone to the organ of Jacobson corresponds essentially to 

 that of the palatine process of the premaxilla in various mammals " 

 (p. 582). 



I may now proceed to state my own observations and conclusions 

 respecting the anatomy of the dumb-bell bone and of the parts 

 related to it. 



In studying series of coronal sections one cannot fail to be 

 impressed with the intimate relations between the dumb-bell bone, 

 on the one hand, and Jacobson's organ and the cartilaginous 

 septum nasi, on the other. I have already referred to Symington's 

 account of the relation to the organ of Jacobson, but he has 

 passed over without special remark the relation of the dorsal 

 moiety of the bone to the nasal septum, a relation so marked as 

 to lead Sir Wm. Turner to take that dorsal portion of the bone 

 as part of the vomer. 



It may, perhajjs, be better to give at once a complete account 

 of the structure and relations of the bone as I have myself 

 determined these both by fresh dissection and by the examination 

 of frontal sections. 



In a specially large adult male Oniithorhynchus measuring over 

 .50 cm. between the tips of the snout and tail, I find the palatine 

 plate of the "dumb-bell " bone to measure 9-5 mm. in total length. 

 This measurement includes about 2 mm. of the posterior extremity 

 of that plate which is overlapped by the palatine cartilage of the 

 nasal floor (see fig. 1, h and d) just in front of the anteriorly 

 directed median process of the osseous maxillary palate. The 



