392 Proceedings of Royal Society of Edinburgh. [sess. 
veloped, and the cartilages are more or less rudimentary. The 
nasal floor cartilage is simple in its construction, and gives rise 
behind to Jacobson’s cartilage and a rudimentary outer nasal floor 
cartilage. There is no turbinal to Jacobson’s cartilage, but there 
are rudimentary indications of its former existence in both groups. 
In the Marsupialia the organ almost invariably opens into the 
upper end of the naso-palatine canal; in the Edentata it opens 
into the nasal cavity considerably in advance of the canal. 
In the Rodentia we have a condition which agrees considerably 
with the Edentata as regards Jacobson’s cartilage and the mode of 
opening of the organ ; hut there is an additional feature present in 
a peculiar development of the outer nasal floor cartilage. This 
cartilage, which is quite unconnected with any other, forms a floor 
to part of the nasal cavity behind the naso-palatine canal, and 
passing forward forms a support to the outer wall of the canal. 
In all the higher Eutheria the organ and its cartilages are formed 
on a common type, with very slight modifications. In this variety 
the peculiarity is due to Jacobson’s organ opening into the naso- 
palatine canal, and to both Jacobson’s cartilage and the outer nasal 
floor cartilage sending forward processes supporting the ducts. 
The organ itself is, as a rule, rudimentary, — at least, when com- 
pared with the lower mammals. 
Erom the consideration of the peculiarities shown, the author 
proposes to divide the Eutheria into two groups, — the former in 
which the nasal floor cartilages are formed on the primitive type, 
and the other in which the cartilages undergo a peculiar and 
characteristic development. The former group, for which the name 
Archeeorhinata is proposed, will include the Edentata, and, most 
probably, the Rodentia. The higher group, for which the name 
Ccenorhinata is proposed, will include the Chiroptera, Insectivora, 
Carnivora, and Ungulata, with most probably the Primates, and 
possibly the Cetacea and Sirenia. 
