238 DR R. BROOM ON THE 



Ornithorhynchus, it will be observed that in the posterior parts of the organ there is a 

 close agreement between the two forms. In Echidna, the whole organ being posterior to 

 the duct, this region is naturally developed to a greater extent than in Ornithorhynchus, 

 where the organ extends in front of the duct as well as behind, but otherwise the only 

 points of difference at present worthy of note are the absence of the prevomer in 

 Echidna, and the different degrees of development of the turbinal plates — in Ornitho- 

 rhynchus large and curved ; in Echidna small and flat. The anterior part of the organ 

 in Echidna differs remarkably from that in Ornithorhynchus, and yet it is not difficult 

 to imagine an intermediate or ancestral form from which both would be derived — the 

 Echidna type by simplification and the Ornithorhynchus type by specialisation. In this 

 hypothetical ancestral form the organ probably extended little, if at all, in front of the 

 duct opening, but had a well-developed turbinal which extended right to the front >f 

 the organ. If in Echidna the turbinal were carried forward to the front of the organ, 

 and the part of the organ in the neighbourhood of the naso-palatine canal, instead of 

 being reduced to a duct, were well developed, there would be as great a resemblance to 

 the Ornithorhynchus condition even in this region as is to be seen in the posterior part. 

 The well-developed anterior part of the organ in Ornithorhynchus is but a continuation 

 forwards of the various structures met with in the region of the duct rendered possible 

 by the great development of the nasal floor cartilage in the anterior region. 



Marsupialia. 



The marsupial organ of Jacobson was apparently first described in 1891 by 

 Symington, who studied the condition in pouch specimens of the kangaroo and rock 

 wallaby. At that time the organ in Echidna was undescribed, and Symington, com- 

 paring the Marsupial organ with that in Ornithorhynchus, concluded that its affinities 

 were more with the higher Eutherian type than with that of the Monotreme. With the 

 exception of a short note by C. Kose on the organ in the wombat and opossum, and a 

 few incidental references in one or two of my own papers, nothing further had been 

 written on the subject till last year, when I communicated a paper " On the Comparative 

 Anatomy of the Organ of Jacobson in Marsupials " to the Linnean Society of New South 

 Wales. This paper, which will appear in the Proceedings of the Society, contains a 

 description of the organ in representative genera of the principal groups, and of the 

 changes met with at different stages of development. From my study of the organ in 

 the various marsupials, it was seen that in all the diverse forms the organ conformed 

 more or less to one main type. Of this main type, however, there are two minor 

 varieties — the simpler one found in the Polyprotodonts, the more complex in the 

 Diprotodonts. In the present paper I will take Dasyurus as the typical representative 

 of the former group, aud Petaurus of the latter. 



Dasyurus. — In Dasyurus we meet with the simplest form of the marsupial organ. , 

 As the structure of the organ and its relations have been described in some detail in the 



