136 ON THE "dumb-bell-shaped" BONE IN ORNITHORHYNCHUS, 



posteriorly is about 5-5 mm.* The plane of the anterior boundary 

 of the internasal passage corresponds pretty accurately with the 

 tip of the median anterior process of the maxillary palate whose 

 upper surface, covered by the nasal mucous membrane, forms the 

 lower boundary of the passage. 



"When the nasal septum is viewed from the side, the internasal 

 aperture is seen to be overhung and almost concealed by the 

 prominent shelf-like ridge running backwards upon the septum, 

 and forming the posterior continuation of the prominence con- 

 taining the organ of Jacobson (see figs 4 and 6-8 gl.). The roof 

 of the internasal passage may be regarded as considerably widened 

 by the projection of this shelving ridge on either side of the 

 septum (fig. 7 gl.). 



From what has already been said of the structui'e of the dumb- 

 bell-shaped bone, it will have become evident that that bone 

 presents on each side a wide and antero-posteriorly elongated 

 sulcus between the lateral portions of its palate plate on the one 

 hand, and the vertical portion of the bone, with its dorsal alee, on 

 the other (see fig. 3). In this sulcus is lodged the greater part of 

 tiie organ of Jacobson, enclosed in its incomplete cartilaginous 

 capsule, for which the bone forms a somewhat semi-tubular osseous 

 investment, the tube being incomplete externally (cf. figs. 9-11). 

 In transverse sections this gives rise to the appearance of the bone 

 being " composed of two crescents with their convexities directed 

 inwards" (Symington), The hollows of these crescents are, 

 throughout their greater part but not entirely, lined by the 

 investing cartilaginous capsules of the organs of Jacobson. The 

 osseous sulcus on each side is of course deepest where both the 

 width of the palatine plate and the spread of the dorsal alae are 

 greatest, ^'.e., in the region of the posterior palatine segment of the 

 bone. It shallows rapidly in front of the naso-palatine foramen 

 through sudden reduction of the alse, and the more gradual reduc- 

 tion of the vertical plate itself (fig. 12). Posteriorly it is continued 



* These measurements are given from the very large specimen chosen for 

 description. In another specimen the length was only about 4 '5 mm. It 

 will be understood that all the measurements given are proportionally large. 



