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Show 1891.] ANATOMY OF ORNITHORHYNCHUS. 579 (10), Guinea-pig (11), Rabbit (12), and Dog (13) it forms a less complete capsule, and in no transverse vertical section is it seen in continuity with the cartilage in the outer wall of the nose. Further, in none of these animals is there any trace of a simple or turbinated septum which is so well developed in the Duck-mole. I am not aware of any published account of its condition in the Marsupialia, but in a young Macropus giganteus which I examined there were no indications of a turbinal, and the general form and position of the organ were very similar to that in the Guinea-pig as figured by Dr. Klein. Professor W . N. Parker (14) has recently shown that the Echidna possesses a largely developed Jacobson's organ which has a well-marked turbinal. So far, therefore, as our present knowledge goes, it appears that it is only in the Prototheria amongst mammals that any arrangement of the cartilage of Jacobson's organ exists by which tbe extent of the epithelial surface lining the organ is increased. In various mammals the cartilage of Jacobson's organ attains a large size, while the lumen of the organ is of microscopic size. This is probably an indication that the sensory part of the organ has degenerated more rapidly than its cartilaginous support. In the Lacertilia and Ophidia the skeletal framework of Jacobson's organ contains a turbinal, and it is in these animals that the organ is generally considered to attain its highest development. Thus Dr. J. Beard (15) writes as follows : - " Indeed an examination of the literature of Jacobson's organ clearly shows that just in the group of animals, the Reptilia, in which alone it can be considered as something more than a rudimentary organ, a thorough comparative investigation is a desideratum " (p. 757). I am inclined to believe that this statement of Dr. Beard will require modification, and that further research will show that Jacobson's organ attains its highest development in the Prototheria. M y own observations amongst the Reptilia are limited to its examination in Anguis fragilis and Pelias berus, in neither of which does it appear to me to be so well developed as in the Ornithorhynchus. The arrangement of the structures forming the soft parts of Jacobson's organ are shown in fig. 1, PI. XLIIL, which represents a transverse section behind the naso-palatine foramen and about the middle of the posterior swelling of the dumb-bell-shaped bone. It will be seen that there is a very marked difference in the structure of tbe soft parts lining the general cartilaginous capsule (a) and that covering the turbinated ridge (b). The turbinal cartilage is covered by some connective tissue, external to this is a thick layer composed mainly of blood-vessels and glandular tissue, while the free surface is provided with a layer of ciliated columnar epithelium. On the other hand, the structures lying on the concave surface of the cartilaginous capsule are evidently nervous. Next the cartilage are numerous small bundles of non-medullated nerve-fibres, which are separated from the sensory cells by a thin layer of connective tissue. In m y specimen this sensory layer contains a large number of oval or rounded cells with distinct nuclei, but there are no |