560 
MAMMALIAN PRENASAL CARTILAGE, 
here an opportunity of seeing the uncomplicated higher mamma- 
lian condition, and though the bat is high in the scale of organ- 
isation it will thus be well to consider it first. If a transverse 
vertical section be made in the plane which passes through the 
opening of Jacobson's organ into Stenson's duct there is seen (fig. 
4) a delicate nasal septum (n.s.) which does not reach the level of 
the nasal floor, with on either side of its base the usual cartilages 
of the nasal floor (n./.c), here distinct from the alinasals. 
Inferiorly Stenson's ducts (n.p.c.) are seen passing up from the 
palate with between them the papilla. Round the upper part of 
the duct is a sickle-shaped piece of cartilage whose inner part, 
surrounding what is practically the anterior part of J acobson's 
organ, is the anterior continuation of J acobson's cartilage, and the 
outer part of which is the similar continuation of Stenson's 
cartilage. Between the portions representing J acobson's cartilage 
is found a small median mass (p.n.) which from its relations may 
almost certainly be regarded as the prenasal. Posteriorly this 
cartilage has no connection with the nasal septum, but it occupies 
a position somewhat similar to that of the prenasal in the foetal 
calf (fig. 7). A very little in front of this plane the small pre- 
nasal is seen sending downwards two lateral plates which pass 
one to each side of the papilla and thus forming its framework 
{p.n. fig. 5). There is no further anterior extension. Though 
the downward lateral processes can hardly be homologous with 
the lateral extensions of the prenasal in Orniihorhynchus, they 
illustrate potentialities of the structure usually latent. The con- 
dition is, however, specially interesting as throwing light on the 
peculiar structures found in the Marsupials. 
In both Diprotodont and Polyprotodont Marsupials there is in 
the anterior part of the palate an unusually well marked papilla 
which generally separates the naso-palatine ducts considerably. 
It is remarkably developed in the Wombat and very well in the 
Phalangers. In all the forms I have examined ( Macropus, Tri- 
chosurus, Das;/urus and Peramelex) this papilla is supported by a 
cartilaginous plate. In the adult Trichosurus there is a median 
ridge from which lateral plates pass off. Here and there the 
