558 
MAMMALIAN PRENASAL CARTILAGE, 
In fig. 1, PL xliv., is represented a longitudinal median section 
of the snout of Ornithorhynchus which shows the relations of the 
cartilages. The nasal septum proper (n.s.) is seen in front to 
divide into the upper delicate cartilaginous process (a.n.s.) and 
the lower well developed prenasal (p.n.). The upper process 
passes between the anterior nares, whose position is indicated by 
the dotted lines (a.n.), and is probably the homologue of the 
anterior part of the nasal septum in the higher mammals, as to it 
are attached, in its hinder part at least, the alinasals. Though 
in the region of the anterior nares it is considerably removed 
from the prenasal plate, on passing forward it approaches the 
latter, and ends in close connection with it, at the point where 
the prenasal becomes arrested in the middle line. This point 
(marked *) probably represents the anterior end of the beak in the 
ancestor of the Platypus, as not only do the two cartilages here 
end together, but this is practically where the two premaxillaries 
would meet if they came together. Connected with it, moreover, 
there is a further feature of importance. On the upper side of 
the beak in the middle line is a small area of thickened epithelium 
(c) unlike that of the rest of the beak. This is probably the 
remains of the caruncle. If it be so, and it is quite perceptible 
externally in the fresh specimen even to the naked eye, it corrects 
the statement of Owen's* that no trace of the caruncle can be 
found in the adult. Whatever be its signification, it is evidently 
closely related to the supposed apical point of the primitive beak, 
as a series of fibrous bands pass from it to the latter. 
In the anterior part of the section is seen the anterior union of 
the incurving lateral portions (pn 1 ), and between this and the 
supposed apical point the cartilage is seen to be absent. 
The same section illustrates a few other points of interest. 
The prevomer or dumbbell-shaped bone (P.vo.) is seen cut longi- 
tudinally. Immediately in front of it Jacobson's cartilage (J.c.) 
may be noticed at the place where towards the anterior part of 
the organ the cartilages meet in the middle line. Behind the 
* R. Owen. " Monotremata." Todd's Cyclop. Anat. and Phys. 
