240 DK R. BROOM ON THE 



Petaurus. — Petaurus I have chosen for consideration, as it shows the diprotodont 

 marsupial characteristics in their most typical form. The main part of the organ agrees 

 very closely with that in Dasyurus, as will be seen by comparing fig. 15 with fig. 11. 

 In front, however, there are some remarkable differences. The nasal region, generally 

 at its anterior part, is somewhat more expanded laterally than in Dasyurus, and the 

 inferior septal ridges are much more developed. It is probably in connection with this 

 broadening out of the base of the septum that the inner part of the nasal floor cartilage 

 assumes its characteristic development. In front of the naso-palatine canal the nasal 

 floor cartilage, as seen in fig. 12, closely resembles the condition in Dasyurus (fig. 8). 

 In fig. 13, in the plane just behind the point where the palatine process divides off from 

 the premaxillary, the nasal floor cartilage is seen in its characteristic form. Its outer 

 part is very much reduced, but the inner part, which becomes Jacobson's cartilage, id 

 much better developed than in Dasyurus. By comparing this figure with fig. 9, the 

 following points of difference will be noticed. Instead of the cartilage being composed 

 of a vertical curved plate, with a short process passing to the inferior septal ridge from the 

 upper end, we have here a moderately flat plate passing markedly outwards, and resting 

 on the palatine process of the premaxillary, while from near its middle there passes 

 upwards and outwards a well-marked process, which passes into and supports the well- 

 developed inferior septal ridge. The interpretation of the morphology of the different 

 structures here seems at first sight rather difficult, but an examination of the succeeding 

 planes gives a satisfactory solution of the problem. In fig. 14 we have represented a 

 transverse section through the point where Jacobson's organ opens into the naso- 

 palatine canal and into the nasal cavity. It will be observed that there is a much more 

 marked connection between the organ and the nasal cavity than in Dasyurus. In the 

 ridge we find a portion of cartilage which we have no difficulty in recognising to be the 

 outer bar of Jacobson's cartilage, and we are thus driven to the conclusion that the ridge 

 support seen in fig. 13 is the homologue of the short plate which passes out to the little 

 ridge in Dasyurus. This being so, we see that two of the peculiarities of the Petaurus 

 arrangement are (1) a great development of the ridge process, and (2) its arising from 

 near the middle of the inner plate instead of from its upper end, which probably means 

 that in Petaurus the upper part of the cartilage is developed to a much greater degree 

 than in Dasyurus. A third peculiarity, and probably the most important, is that the 

 inner plate of the cartilage at its lower end passes down on the outside of the pain tine 

 process of the premaxillary, and forms the inner wall of part of the naso-palatine canal. 

 In almost all other respects the details of the anatomy in Petaurus agree with those in 

 Dasyurus. One feature worthy of note is, that in the diprotodonts the organ almost 

 always has a well-marked vascular plexus, while in the polyprotodonts the plexus is 

 more or less rudimentary. In my paper above referred to I have pointed out the 

 peculiarities of the different diprotodonts, and here need only call attention to one, viz., 

 yEpyprymnus. In this rat-kangaroo we have a peculiar arrangement found in no other 

 marsupial yet examined, namely, that the organ opens directly into the nasal cavity 



