G04 THE ORGAN OP JACOBSON IN MARSUPIALS, 



more than lialf the diameter, while the lumen is slightly crescen- 

 tic, owing to the outer wall being much better developed at its 

 central than lateral portions. 



Pseudochirus peregriiius, Bodd., (adult), Petauroides volans, 

 Kerr, (adult), and Petaurus breviceps, Waterh., (adult). These 

 three genei^a agree with each other so markedly that it will only be 

 necessary to describe the condition in one — Petaurus — and call 

 attention to the points in which the others differ from it. 



In a plane immediately in front of the papilla, the condition of 

 the nasal-floor cartilage is found to agree very closely with that 

 described in Perameles, each inner end having an ascending plate 

 closely placed against the sides of the base of the sej^tum. The 

 only mai'ked difference is that the lateral part of the cartilage is 

 much curved; this, however, is rendered necessary by the largely 

 developed first incisors. In the plane passing through the middle 

 of the papilla the inner ascending j^late of the nasal-floor cartilage 

 is much shorter, but has become broadened out, while the inferior 

 septal ridge, which anteriorly was developed considerably verti- 

 cally, is here a much more defined ridge, and from the outer angle 

 of the irregular square-shaped inner part of the nasal-floor 

 cartilage a slight process passes into the ridge. The outer part of 

 the nasal-floor cartilage becomes almost entirely lost. PI. XLiv. fig. 

 10 represents a section through the third incisor or the posterior 

 part of the papilla. Here the nasal-floor cartilage assumes an 

 appearance which may be regarded as typical of the Phalangers. 

 The inferior septal ridge is removed from any direct connection 

 with the septum, and the process from the inner part of the nasal- 

 floor cartilage (which may even here be regarded as Jacobson's 

 cartilage) supporting it, instead of coming from the inner part of 

 the cartilage, springs from a point considerably farther out, while 

 an independent continuation of the nasal-floor cartilage extends 

 on to the base of the septum. In Petaurus Jacobson's cartilage 

 lies very obliquely outwards on the palatine process, but in 

 Petauroides and Pseudochirus the cartilage is much more vertical 

 (c/. fig. 4); otherwise, however, the structures are similar. 

 Inferiorly the cartilage plate extends downwards considerably 



