ly91.] 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 know- 

 ledge goes, it appears that it is only in the Prototheria amongst 

 mammals that any arrangement of the cartilage of Jacobson's organ 

 exists b)- which the 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 theLaccrtilia 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 Eeptilia, 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. 

 My own observations amongst the Reptilia are limited to its ex- 

 amination in Anguis fragilis and Pelias berus, in neither of which 

 does it appear to me to be so well developed as in the Ornitho- 

 rhynchus. 



The arrangement of the structures forming the soft parts of 

 Jacobson's organ are shown in fig. 1, PI. XLIII., 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 the soft parts lining the general cartilaginous capsule (a) and 

 that covering the turbinated ridge ib). The tnrbinal 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 my specimen this sensory layer contains a large number 

 of oval or rounded cells with distinct nuclei, but there are no 



