608 THE ORGAN OF JACOBSOX IN MARSUPIALS, 



Jacobson's organ makes its appearance between the outer end of 

 the ridge process and the inner plate of Jacobson's cartilage, 

 dividing the one from the other; but though the outer part of the 

 ridge process — clearly the outer bar of Jacobson's cartilage — 

 becomes detached from the inner plate, it still retains its connec- 

 tion with the outer part of the nasal-floor cartilage. In PI. XLVi. 

 fig. 4, however, — a little further back still — the outer bar is free 

 from the nasal-floor cartilage which is now lost. On this plane 

 the aj)pearance quite agrees with that in the Ringtail — the organ 

 connecting with the naso-palatine canal in quite a similar way. 

 In PI. XLVi. fig. 5 the organ is closed, and the naso-palatine cniial 

 is merged in the nasal cavity. In the following figure the usual 

 appearances are presented. The inner plate of Jacobson's cartikxge 

 has united below with the outer bar, and an irregular U-shaped 

 hollow is formed for the reception of the organ. 



The organ is large and has an irregular crescentic shape; with 

 a well developed sensory wall. The hilus is large and contains 

 two or three large veins and one or two small; while all along the 

 outer side of the organ is an enormous amount of glandular 

 tissue, in which it differs from that of the other Phalangers. 



Subfamily P H A s c o L A R c T l N .E. (Plate XLVi. figs. 7-9.) 



Phascolarctus cinereus, Goldf., (two-thirds grown). In Phascol- 

 arctus we have a very highly modified type which differs in 

 many ways from that of the Phalangers just described. 



The naso-palatine canal is ver}' long and oblique. In PL XLVi. 

 fig. 7 we have represented a section through the plane a little in 

 front of the point where the premaxillary gives off its palatine 

 process. In this and the following sections the most striking- 

 peculiarity is the depth of the secondary palate. The nasal-floor 

 cartilage is well developed, but Avith the narrowing of the nasal 

 cavity only a very small portion is really a floor. At its inner 

 end it is very simple and abuts against the base of the septum. 

 Below the septum will be seen the vomer, a most exceptional 

 occurrence, this being the only Marsupial known in which the 

 vomer is directly in contact with the body of the premaxillary. 



