Witson anp Marrin— Zhe Anatomy of the Muzzle of Oruithorhynchus. 187 
be associated with the close approach to the region of the nostril which opens dorsally 
by breaking through the alinasal. At this level, too, the maxillary splint of the 
rostral crus has disappeared. 
In fig. 10 (see Ime of section in fig. 17) it is to be observed that the prenasal 
plate has acquired an attachment to the imner border of the premaxilla, and hence 
it here forms a complete inter-crural plate of cartilage. It is at this plane then that 
the prenasal cartilage is first seen to be united by the medium of the premaxilla with 
the superior marginal (“labial”) cartilage with which we surmise it to have been 
originally directly continuous laterally, as it is permanently in front. The last trace 
of the nasal splint of the rostral crus is seen in this section, and here in the immediate 
vicinity of the external nostril the horizontal septa (4.f.) of the nasal cavity lose their 
continuity between the lateral walls. 
Fig. 11 passes through the hinder border of the external nostril (azz. nar.), and 
only mere traces of the horizontal septa projecting from the mesial wall of the cavity 
are visible. The dorsal moiety of the septum is seen to have flattened out and to 
have descended somewhat, but it still remains in continuity on each side with the 
mesio-dorsal portion of the alinasal, which here helps to form the skeleton of the 
nostril on the inside. 
In fig. 12—a section through the middle of the apertures of the external nostrils 
—a somewhat odd arrangement of cartilage is seen mesially between the nostrils, due 
to the further descent of the remnant of the dorsal portion of the septum nasi (w.s.d.) 
and its flattening out, and to the elevation, on each side of it, into a vertical attitude, 
of the remains of the mesio-dorsal portion of the alinasal (a.v.). This rearrangement 
occurs rather abruptly, and at first we were somewhat puzzled by the box-like 
structure thus formed. A comparison of figs. 11 & 12, however, renders the 
structural change quite intelligible. 
Further forwards, as in fig. 13, which represents a section through the anterior 
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border of the nares, the components of the mesial cartilaginous structure just 
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described separate from one another and increase in size, and, in fig. 14, the mesial 
part of the alinasal once more becomes continuous with the lateral portion, completing 
the cartilaginous wall of the nostril below and in front. (See also fig. 15. 
t=) 5 
Fig. 16 (see line of section in fig. 17) shows a section taken through the very 
anterior tips of the premaxillary bones, exhibiting the actual continuity of the 
prenasal plate with the marginal cartilage of the jaw. It is to be observed that the 
prolongation forwards of the dorsal moiety of the septum nasi is still visible, also 
forming a “prenasal” cartilage (z.s.¢.). It lies immediately dorsal to the prenasal 
plate, and a little more anteriorly it vanishes, while the prenasal plate becomes the 
“valance of solid hyaline cartilage” in front of the bony rostrum (fig. 17, s.7z.c.). 
