186 Macteay Memoria Vonume. 
In fig. 4 (see line of section in fig. 17) the septum nasi is seen to have undergone 
separation into a ventral moiety (z.s.v.), still more expanded laterally than in the 
preceding sections, and a dorsal moiety (v.s.d¢.) continuous at its dorsal border with 
the alinasal cartilages. The ventral borders of the alinasal cartilages have become 
more flattened, being encroached on by the enlarging ventral part of the septum nasi. 
The very anterior end by the dumb-bell-shaped bone is cut through at this plane 
and, being slightly notched anteriorly, it appears in two pieces. An interruption in 
the continuity of the alinasal cartilage dorsally 1s noticeable. 
In fig. 5 the only important feature is the shght separation from each other of 
the hitherto apposed mesio-ventral borders of the alinasals, thus exposing the 
expanded ventral part of the septum nasi from below. 
In fig. 6 the separation of the alinasals ventrally is well marked, while it is to be 
noted that here the ventral moiety of the septum nasi has not only become widely 
triangular but has descended somewhat, so as to occupy a position between, and in the 
same plane as, the divaricated alinasal cartilages. (See also fig. 17, s.z. and a.z.) 
Fig. 7 shows how, still more anteriorly, the ventral division of the septum nasi 
fuses laterally with what was formerly the mesio-ventral border of the alinasal, so as 
to constitute a continuous floor-plate of cartilage whose composite character must 
not be lost sight of. It is to this cartilaginous plate that we have applied the name 
of “prenasal plate” (f.7..), in view of the fact that its mesial element is a forward 
prolongation of the ventral part of the cartilaginous nasal septum, z.e., of the axial 
continuation of the primitive chondrocranium. 
Again we would lay considerable emphasis on the great similarity, which seems 
to us to amount to actual morphological identity, between this condition in Platypus 
and that which the late Professor W. K. Parker (21) has figured in the illustration 
of his memoir already referred to (his Pl. 11. fig. 1, and also fig. 5). 
If Parker’s figure of the skull of an embryo Cygaas nigricollis, viewed from the 
ventral aspect, be examined along with the view of the sagittal section in his fig. 5, 
and compared with our figure 17, it will be seen that, as im Platypus, so here, the 
nasal floor posteriorly is formed by the alinasal cartilages meeting in the mesio-ventral 
line, but that as they are traced anteriorly they are seen to diverge from one another 
while the prenasal expansion of the ventral border of the septum descends into the 
palate and takes up a position between them, and then extends forward as a prenasal 
plate between the developing premaxille. In Platypus, of course, the dumb-bell- 
shaped bone partly clothes the ventral surface of the apposed alinasals; and anteriorly 
the almasals are continuous on either side with the broad prenasal plate. 
Fig. 8 shows little more than fig. 7. It may be noted, however, that the 
continuity of the alinasal is further interrupted laterally. This change is probably to 
