BY R. BROOM. 605 



past the lower edge of the palatine process, a condition more 

 apparent in Pseudochirus than in Petaurus. On passing back- 

 wards the outer part of the cartilaginous process of the ridge 

 becomes detached as the outer bar of Jacobson's cartilage. In 

 PI. xLiv. fig. 1 1 the anterior part of Jacobson's organ is indicated, 

 with the naso-palatine canal connected with the short duct of the 

 organ. In PL XLiv. fig. 12 the organ communicates freely with the 

 nasal cavity at the plane where the naso-palatine canal becomes 

 part of the cavity. 



From PI. XLIV. iigs. 5 and 6 it will be seen that in Pseudochirus 

 the opening of the organ is more directly into the upper part of 

 the canal, while in Petauroides (fig. 8) the condition agrees 

 more nearlj' with that in Petaurus. The difference, however, 

 is only a very slight one of degree. 



After the closing of the organ the lower part of Jacobson's 

 cartilage unites with the outer bar in the usual manner. In 

 Pseudochirus the ridge is considerably lower than in the other 

 Phalangers, so that when the lower part of Jacobson's cartilage 

 is complete, instead of an irregular U-shaped aj^pearance we have 

 a very i-egular L, as in PL xliv. fig. 7. In Petauroides (fig. 9) 

 the cartilage has the more usual appearance. 



The organ in all these genera is well developed, and has on 

 section a rather elongated kidney shape. In the small Petaurvis 

 the sensory wall is larger proportionally than in the other two 

 genera. The hilus is very broad and only but' slightly depressed, 

 leaving a larger lumen to the organ. In all three genera there is 

 a distinct venous plexus usually composed of one, two, or three 

 A'essels anteriorly, which branch into six or more posterioi'ly. 

 There are but few glands in connection with the organ, except at 

 the posterior part. 



Trichostiriis vulpecula, Kerr, (mammary foetus, head length 

 7 "5 mm.). In this very small mammary foetus, which may be 

 taken as the size at birth, the cartilages ai-e all fairly well 

 developed, and the ossification of the premaxillary bones quite 

 distinctly marked. In the plane of the developing incisors the 

 nasal-floor cartilage is very well developed, as seen in PL xlv. fig. 1. 





