600 THE ORGAN OF JACOBSON IN MARSUPIALS, 



its support. In the immediately succeeding planes the innei' plates 

 of the nasal-floor cartilages about to become Jacobson's cartilages 

 are seen approaching somewhat and the palatine processes becom- 

 ing moi'e curved along their inner sides ; while the process of 

 cartilage supporting the ridge becomes a detached bar. This bar 

 thus becomes detached further forward than in either Dasyurus 

 or Didelphys. A very short distance behind the plane of the 

 posterior part of the papilla, the naso-palatine canal is found 

 passing inwards below the lower edge of Jacobson's cartilage and 

 even below the lower edge of the palatine process. From this 

 point it passes outwards, upwards, and slightly forwards into the 

 hollow of the lower half of Jacobson's cartilage, where it meets a 

 short but distinct Jacobson's duct. It also passes outwards and 

 backwards, as seen in fig. 2, opening into the nasal cavity. On 

 this plane the short duct of Jacobson is replaced by the lower 

 part of the organ projDer, which is almost shut off from the naso- 

 palatine canal. In the relations of the canal to the lower part 

 of the palatine process and of the cartilage of Jacobson there is a 

 marked agreement with Didelphys, though the lower unsupported 

 part of Jacobson's cartilage is much greater here than in that 

 genus, and cleai^ly suggest the development met with in both the 

 Phalangers and the Kangaroos. Almost immediately beyond the 

 plane of the closing of the organ the lower end of the inner plate 

 of Jacobson's cartilage curves round and unites with the outer 

 bar, giving on section the usual TJ-shaped hollow trough. 



The organ itself closely resembles that in Didelphys in the 

 folding together of the feeble outer wall. Thei'e is, however, a 

 marked diffei-ence in the support the cartilage obtains from the 

 palatine process. In Pex'ameles the palatine process is largely 

 developed, and forms a bony support to almost the whole inner 

 and lower sides of the cartilage. About the middle region of the 

 organ, in fact, with the exception of a very small portion at the 

 upper angle, the palatine process not only completely suri-ounds 

 it, )jut at its outer edge even replaces the cartilage. On nearing 

 the posterior end of the organ the cartilage becomes comi:)letely 

 lost in the whole lower region being replaced by the jjalatine 



