61<S THE ORGAN OF JACOBSOX IN MARSUPIALS, 



any very deep significance, as in the Mouse there is but a single 

 hilar vessel, while in the allied Guinea-pig there is a regular 

 jjIcxus. Still it is intex'esting to note that the plexus is constant 

 anuing the Phalangers, so far as known. The arrangement of 

 mucous glands is very vai'iable anteriorh-; in Petaurus, Pseudo- 

 chirus and Petauroides they are absent or scanty, while in 

 Trichosurus they are abundant. As already observed, this is a 

 point of little importance. 



In Phascolarctus, not having examined the early conditions of 

 the jDarts, it would be rash to say much on the relationships of 

 the organ. Appai'ently the adult organ and cartilage differ ^ ery 

 considerably from those in the Phalangers. Its most interesting 

 points are — (1) the large proportional development of the nasal- 

 floor cartilage; (2) the low position relative to the cartilage of 

 Jacobson occupied by the palatine process ; (3) the anterior 

 de^'elopment of the vomer; (4) the persistence of the cartilaginous 

 roof; (5) the complete tube formed by Jacobson's cartilage; and 

 (6) the presence of a plexus on the outer side of the organ. 

 Whether as a parallel development or as indicating an affinity it 

 is difficult to say, but there is a very decided resemblance in man}- 

 ways to the condition in the Wombat. 



In the Macropods, though there are features of resemblance to 

 the Phalangers, both the ascending and descending processes of 

 the inner parts of the nasal-floor cartilage are less marked. In 

 Macropus the descending process is due more to a bending down 

 of the nasal-floor cartilage than to a distinct downgrowth, though 

 in ^pyprymnus the downgrowth though short is quite distinct, 

 at least in the foetus. In Macropus the relations of the naso- 

 palatine canal to the opening of the organ and the nasal cavity 

 follow the usual type. In i^Epyprymnus, however, there is, with 

 practically no difference in other details, the i^emarkable and, so 

 far as my studies go, unique condition of the organ opening out 

 to the anterior nasal floor, and not into the naso-palatine canal. 

 This is practically the condition which we find in an extreme degree 

 in the Rodentia. If the section (PI. xlvii. fig. 11) illustrating 

 the condition in the Rat-Kangaroo be compared with the similar 



