Anatomy of Notorydes typJdops. 77 



claims that, on account of the very slight tendency of these 

 parts to vary with external variations, " we have a factor of 

 considerable value in the classification of the Eutheria, probably 

 of more value than eitlier dentition or placeutution." If this be 

 so, and there seems strong evidence in its favour, we ought to 

 tind in this organ data on which to base a true conception of the 

 relationships of such an aberrant form as Notoryctes, especially 

 valuable since its embryology remains at present unknown. In 

 itself a desirable result, this should also assist in defining the 

 affinities of associated groups. Heretofore, apparently, nothing 

 has been known of its structure in Notoryctes, nor even of its 

 presence. This being so, it was suggested when working out the 

 relations of the naso-lachryraal duct in connection with the eye, 

 that I should include Jacobson's Organ in this research. Further- 

 more, we find that the structure and relations of the cartilages 

 and bones associated with the nose are well worthy of record, as 

 well as those of the organ itself. 



In Broom's valuable thesis on Jacobson's Organ, ^ he has dis- 

 tinguished four types of this structure in mammals corresponding 

 in part to the main groups : Monotreme, the most highly 

 developed, Marsupial, Rodent, and general Eutherian ; the main 

 features of distinction being the character of the connection of 

 the lumen of Jacobson's Organ with the naso-palatine canal, or 

 with the nasal cavity, and tlie arrangement and degree of 

 complexity of the cartilages. In view of the apparent value of 

 this organ in classification, and the much modified character of 

 Notoryctes, I have thought it desirable to make a more complete 

 comparison of the various details of structure, with similar parts 

 in other forms, than might have been necessary in some other 

 animals. 



Posit loti of the Organ of Jacobson. 



The organs of Jacobson are, as stated above, well developed in 

 Notoryctes, being approximately equal in size to those of the 

 Rabbit. They are situated near the floor of the nasal cavity, one 

 on either side of the median line, just in front of the vertical 

 plane of the osseous nasal septum, i.e., 3.6 to 3.8 mm. from the 

 anterior edge of the snout. They are separated from each other 



1 Trans. Roy. Soo. Edin., vol. xxxi.x., 1898-1900, p. 234. 



