Wison and Martin—The Anatomy of the Muzzle of Ornithorhynchus. 185 
it is in the bird, for we have not merely in the adult the persistence of the broad 
inter-crural plate but of that great “leafy tract” of inarginal cartilage extending 
forwards and laterally round the free margin of the bony rostrum. 
It will thus appear that the existence of a true cartilaginous homologue to the 
prenasal of the pig is an additional and sufficient reason for rejecting the theory that 
the latter is represented in Platypus by the dumb-bell-shaped bone. 
We believe that the topographical and other relations of the structures under 
consideration will be best elucidated in detail by our commenting seriatim upon the 
more important features presented by series of coronal sections, of which the figures 
represent the most typical and significant specimens. 
A coronal section of the snout of Platypus through the anterior end of Jacobson’s 
organ is represented in fig. 1. It closely corresponds to the illustration fig. 1 of 
Symington’s paper (20). 
Here we have to note the constitution of the cartilaginous nasal skeleton as 
formed by the septum nasi (s.7.) thickened along its ventral border and in contact 
ventrally with the approximated ends of the alinasal cartilages (@.2.) which here are 
thickened so as to form cartilaginous capsules for the organs of Jacobson (/). These 
alinasal cartilages completely surround the nasal cavities forming the floor, lateral 
walls and roof of the capsules, and dorsally are seen to be continuous with the dorsal 
border of the septum. Ventrad of the alinasals, and closely applied to them, where 
they meet below the septum nasi, is the anterior expanded portion of the dumb-bell- 
shaped bone (d.0.). 
In the lateral region of the section is seen the rostral crus cut across and 
consisting of its three bony elements—the premaxilla (f.2.) forming the chief 
constituent, supported internally and externally by the rostral processes of the nasal 
(z.) and superior maxilla (vzxv.). Wedged into a groove on the outer border of the 
crus between the edges of maxilla and premaxilla is the base of the superior marginal 
eartilage (s.7.c.) which extends outwards, thinning towards the free hip-margin (curled 
slightly in process of hardening). 
For the characters of sections taken posterior to this coronal plane we refer to 
Symington’s paper already quoted. 
The subsequent figures represent the successive changes in configuration of the 
sections seen in passing forwards from the plane of fig. 1. Figs. 2 and 3 represent 
stages of transition from the foregoing type of structure. They are taken in front of 
the organ of Jacobson and the ventral borders of the alinasal cartilages have thus 
diminished in bulk. The ventral border of the septum nasi is seen to be undergoing 
slight lateral expansion, while the intermediate part of it immediately above is 
becoming constricted. 
Z 
