BY J. T. WILSON. 137 



backwards beyond the posterior segment of the palate plate of the 

 bone as a groove upon the side of tlie posterior spur, where that 

 lies in the roof of the internasal passage (figs. 7 and 8). 



As Symington has shown {loc. cit. cf. his pi. XLlli. fig. 1) the 

 organ of Jacobson in Ornithorhynchus, with its cartilaginous 

 capsule, is continued forwards for some little distance in front of 

 the naso-palatine foramen into which its duct opens, and in fact it 

 reaches to near the anterior end of the preaxial segment of the 

 dumb-bell ending immediately in front of the anterior commence- 

 ment of the vertical crest. Posteriorly the organ ends at the 

 })lane of the anterior margin of the internasal aperture, i.e, that 

 of the anterior limit of the maxillary palate, and only a very short 

 distance behind the plane of the hinder end of the dumb-bell- 

 shaped plate. 



The outer wall of its cartilaginous capsule, however, is continued 

 a little further back, closing in externally the continuation, on 

 the posterior spur, of the lateral groove of the bone, in which are 

 lodged the great nerves and the vessels destined for Jacobson's 

 organ. These are contained in the base of the shelving ridge 

 referred to above, but the mai'ginal part of the shelf consists merely 

 of mucous membrane, including an elongated mass of glandular 

 tissue. In sections through the middle of the internasal aperture 

 (tig. 7) the glandular tissue is responsible for about two-thirds of 

 the entire width of the shelf, but at the anterior margin of the 

 passage it forms only about a half of the total width. This 

 definite glandular prominence lies outside the capsule of Jacobson's 

 organ, and may be traced forwards as an extra-capsular prominence 

 on its dorso-lateral wall almost to the anterior end of the organ. 

 A small amount of glandular tissue is, however, to be found 

 within the capsule of the organ. 



In the light of these observations I propose now to reconsider 

 the grounds upon which the dumb-bell bone has been so confidently 

 regarded as simply the inner or mesial palatine portion of the 

 intermaxillaj. The criteria which, in Sir Wm. Turner's opinion, 

 were sufficient to establish this judgment, have already been 

 quoted, and we have seen that the first of these depends on an 



