In this paper the author confines his attention to the relations of the organ, and especially to the structure of Jacobson's cartilage, the nasal floor cartilage, and the different developments to which they give rise. From the study of a large series of forms he has been led to the following general conclusions:—(1) That the organ is generally well developed in the primitive groups, and feebly developed in the higher forms; (2) that among allied animals the organ is, as a rule, better developed in the small species than in the large; (3) that in the various members of each Natural Order the organ is constructed on a common type, which admits of only very slight variation; and (4) that, even when the organ is quite absent in certain members of an Order, the nasal floor cartilages are constructed on the same type as in those members in which the organ is developed.

From the very slight tendency there is in the organ and its cartilaginous adjuncts to vary with changing habits, the author concludes that the anatomy of the region affords a valuable factor in classification. A study has been made in most of the Mammalian orders, and the relationships and inter-affinities of the different types have been critically examined.

In the Monotremata the organ is very well developed. The simple nasal floor cartilage in front gives rise behind to Jacobson's cartilage and a well-developed outer nasal floor cartilage, which latter passes inwards and supports Jacobson's cartilage. In this order Jacobson's cartilage differs from all other mammals, and resembles lizards in forming a well-marked turbinal process.