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Show 578 DR. J. SYMINGTON ON THE [Nov. 17, especially in the Prototheria, is, therefore, a subject of considerable morphological interest. M y 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 processes, 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 XLIIL). 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 cartilage 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. W e 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 separate 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 partially 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 XLIIL), but behind the opening it is turbinated, curving downwards and inwards (see fig. 2, Plate XLIIL ; and fig. 1, Plate XLIV.). The cartilaginous framework of Jacobson's organ in the Ornithorhynchus will be found to differ considerably from that of any of the Eutheria in which it has been described. Thus in the Sheep |