578 DR. J.SYMINGTON ON THE [Nov. 17, 



especially in the Prototheria, is, therefore, a subject of considerable 

 morphological interest. 



My series of coronal sections pass through the entire organ, which 

 fortunately was sufficiently well preserved to admit of a satisfactory 

 microscopic examination. 



As there is no distinct external nose and the anterior part of the 

 nasal cavity is not complicated by the presence of turbinated pro- 

 cesses, we should expect the cartilaginous framework of the nose of 

 the Ornithorhynchus to be comparatively simple, and such will be 

 found to be the case. Thus a coronal section through the nose about 

 midway between the nostrils and Stenson's ducts shows a central bar 

 of cartilage forming the nasal septum. This cartilage divides above 

 into two lateral plates, each of which first curves outwards, then 

 downwards, and lastly turns inwards to end, in close contact with 

 the one of the opposite side, just below the nasal septum. It is in 

 the somewhat thickened inner extremity of this lateral plate that 

 the anterior part of Jacobson's organ is embedded (see (c) on fig. 1 

 of Plate XLIII.). A little farther back the cartilaginous floor of the 

 nose is perforated by the naso-palatine foramen or Stenson's duct. 

 This foramen passes from the mouth into the nose on the outer side 

 of Jacobson's organ, while the cavity of the organ itself opens on the 

 inner wall of the foramen. These relations of the naso-palatine 

 foramen and the duct of Jacobson's organ are those which Herzfeld 

 (9) found to exist in the majority of mammals. The piece of carti- 

 lage lying to the inner side of the naso-palatine foramen forms the 

 cartilaginous framework of Jacobson's organ, and in all coronal 

 sections, from the level of the foramen backwards to the posterior 

 end of the organ, its cartilage is seen as an independent mass having 

 no direct connection with either the nasal septum or the cartilage in 

 the floor of the nose. We thus see that the cartilage of Jacobson's 

 organ in this animal is continuous, in front of the naso-palatine 

 foramen, with the cartilage in the floor of the nose, while behind 

 this it appears in coronal sections as a sep irate structure. With the 

 exception of the parts at the naso-palatine foramen and near its 

 posterior extremity, the cartilage forms a complete investment for 

 Jacobson's organ. At the naso-palatine foramen it is perforated for 

 the passage of the duct from Jacobson's organ into the foramen, 

 while posteriorly it is open for the passage into it of branches of the 

 olfactory nerve and of glandular tissue. The cavity enclosed by the 

 cartilaginous capsule is jiartially divided into two compartments by 

 a bar of cartilage which passes from the outer side upwards and 

 inwards towards the inner wall. This septum, which is continuous 

 with the cartilaginous capsule, varies in its form at different levels. 

 Thus in front of the naso-palatine foramen the septum is nearly 

 straight (see fig. 1, Plate XLIII.), but behind the opening it is 

 turbinated, curving downwards and inwards (see fig. 2, Plate XLIII. ; 

 and fig. 1, Plate XLIV.). 



The cartilaginous framework of Jacobson's organ in the Ornitho- 

 rhynchus will be found to diff'er considerably from that of any of 

 the Eutheria in which it has been described. Thus in the Sheep 



