COMPARATIVE ANATOMY OF THE MAMMALIAN OEGAN OF JACOBSON. 239 



above-mentioned paper, a brief account will here suffice. The nasal floor cartilage is 

 well developed and simple in its structure. Fig. 8 represents a transverse section just 

 in front of the naso-palatine canal. Here the nasal floor cartilage is seen as a curved 

 plate of cartilage resting on the premaxillary bone, and with its inner end curved 

 upwards by the side of the base of the septum, and then slightly outwards, forming a 

 support to the inferior septal ridge. The naso-palatine canal on passing upwards passes 

 slightly backwards, and in fig. 9 it is seen cut across as it lies between the premaxillary 

 and its palatine process, and dividing the nasal floor cartilage into an inner and an outer 

 part. The inner part, which becomes Jacobson's cartilage, is very irregularly " C "- 

 shaped, there being an inner curved portion for the reception of the organ and a well- 

 marked little process passing out into the septal ridge. In fig. 10 the canal is found 

 first opening into the anterior end of Jacobson's organ, and then communicating with 

 the nasal cavity. 



If the three sections of Dasyurus figured be compared with the corresponding figures 

 representing the condition in Echidna, the wonderful agreement will be at once evident — 

 the simple condition of the nasal floor cartilage, its division into two parts by the naso- 

 palatine canal, and the mode of communication of the canal with the anterior end of 

 Jacobson's organ and with the nasal cavity. Even the details of structure of Jacobson's 

 cartilage are exceedingly instructive. The outer end of the upper part of Jacobson's 

 cartilage is in fig. 10 seen detached from the inner plate, forming a bar along the 

 concavity of the organ. A very short distance, however, behind the plane of fig. 10 

 the lower end of the inner plate of Jacobson's cartilage passes outwards and unites with 

 the bar. This interesting little bar of cartilage, which is present in nearly all Marsupials, 

 1 have elsewhere called the " outer bar of Jacobson's cartilage." Its chief interest lies in 

 the fact that there is not .the slightest doubt but it is exactly homologous with the 

 rudimentary turbinal found in the anterior part of the cartilage in Echidna, and that it 

 is thus the remnant of an ancestral turbinal. The posterior and main part of the body 

 of the organ lies in an open curved plate of cartilage, which only supports the organ on 

 its inner and under sides. We thus have in this primitive marsupial type a series of 

 structures which are in all details easily traceable to the monotreme condition. In 

 Ornithorhynchus the ancestral monotreme type is complicated by excessive cartilaginous 

 developments, while in Echidna the primitive condition is obscured by the rudimentary 

 condition of the cartilage in the anterior region. In Dasyurus, on the other hand, we 

 probably have a nearer approach to the proto-mammalian proportions than in either of 

 the existing monotremes, but, unfortunately, the organ has become much more feebly 

 developed, and the cartilages are so reduced that at first sight one fails to observe the 

 traces of their former grand developments. 



Didelphys agrees very closely with Dasyurus as regards its organ and cartilages ; 

 Perameles, on the other hand, while agreeing fairly well with these more normal 

 polyprotodonts, shows some slight peculiarities, which have been considered in my above- 

 mentioned paper. 



VOL. XXXIX. PART I. (NO. 8). 2 



