606 THE ORGAN OF JACOBSON IX MARSUPIALS, 



At its inner end it sends up a process by the side of the septum, 

 which latter at this eai'ly stage descends down Ijetween the inner 

 ends of the premaxillaries. At the outer ends the nasal-floor 

 cartilage unites with the alinasal. On reaching the plane of the 

 papilla the nasal-floor cartilage divides into its inner and outer 

 parts; before dividing, however, the downward process of the 

 inner part makes itself manifest. On the left side of PI. xlv. fig. 2, 

 representing the plane a little behind the division of the nasal- 

 floor cartilage, Jacobson's cartilage is seen as a curved plate with, 

 near the middle of the inner concave side, the developing palatine 

 process, present as a minute spicula of bone. The downward 

 process, it will be seen, is more marked than in the young Psendo- 

 chirus. The naso-palatine canal has the usual relations, opening- 

 first into Jacobson's organ and then becoming merged in the 

 nasal cavity. The organ is present as a small oval tube with the 

 inner wall considerably thicker than the outer. 



Trichosurus vulpecula, Kerr, (mammary fcetus, head length 

 10'5 mm.). In this more developed mammary foetus the relations 

 of parts are better seen. In PI. xlv. fig. 4 is shown the complex 

 structure of the inner part of the nasal-floor cartilage just before 

 division. From this figure it will be seen that the descending- 

 process is a structure superadded to the simple nasal-floor cartilage 

 as seen in the Dasyure. The same can probably also be said of 

 the internal ascending process. In PI. xlv. fig. 6 Jacobson's 

 cartilage is an almost vertical plate with the rod-like palatine 

 process along the middle of the inner side. The organ is here 

 very large. 



IVichosurus vuljyecula, Kerr, (mammary fcetus, head length 

 20 mm.). In the series of sections from this specimen we have 

 the steps intermediate between the condition in the early fostus 

 and the adult. The nasal-floor cartilage before division as seen in 

 PL XLV. fig. 7 may be compared with PL xliv. fig. 4, illustrating 

 the similar part in Pseudochirus. The only marked difference is 

 due to the unusually well developed posterior outer part of the 

 nasal-floor cartilage. In the Ringtail the outer nasal-floor carti- 

 lage is only a rudiment, but here it is larger than the inner part. 



