Macteay Menmoriat Vouume. 
bat 
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vax 
In view of the fact that we have not as yet been able to investigate the develop- 
ment of this prenasal cartilaginous tract it would be rash to deny the possibility that 
the marginal cartilage of the upper lip may arise as a truly separate labial cartilage 
which subsequently fuses with the prenasal axial tract behind it. But, (a) the inter- 
crural lamella of cartilage at least is certainly a greatly expanded true prenasal ; (b) 
there is complete continuity in front between this true prenasal and the marginal 
cartilage of the adult upper jaw: and, (c) it is less difficult to imagine an extraordinary 
outgrowth in the lip of a pre-existing expanded prenasal slab of cartilage than to 
inagine the reappearance, or survival, in a single mammal of such an archaic feature 
as the superior labial cartilage for whose archetype we must “go down amongst the 
lower cartilaginous fishes.” 
Even apart from the ample development of the prenasal or rostral cartilage in 
some mammals, ¢.g., the pig (c/ Parker and Bettany, Morphology of the Skull, 1877, 
fies. 77-82), the great development of the corresponding cartilaginous tract amongst 
some birds must be taken account of, the more especially that the avian affinities of 
monotremes are not to be wholly ignored. 
The process of development in the bird’s skull, as described by the late Prof. W. 
K. Parker (19) for the duck tribe, appears to us to present a close and instructive 
analogy to that which we suppose to have occurred in the case of the skull of 
Platypus. 
In reference to the prepituitary portion of the chondrocranium Parker remarks, 
*. . in the bird, as in cartilaginous fishes, that which forms the middle part 
of the foundation of the [orbitonasal] wall is carried forwards as a long ‘ prenasal 
rostrum.’ This part forms most of the skeleton of the skate’s large beak; in the 
bird it 1s the temporary model on which the secondary facial skeleton rs formed.’ And 
again (p. 10), “The morphology of the ‘prenasal rostrum’ is evident ; it is a mere 
fore-growth of the intertrabecula, and in skates, sawfish, and even in some Ganoids 
it is often of great length.” 
The author then refers to his Pl. un fig. 1, representing the ventral aspect of the 
skull of a nearly ripe embryo of Cygnus nigricollis, and to this plate we also would 
specially direct attention. There, in the anterior part of the beak, is seen the carti- 
laginous prenasal rostrum forming a considerable tract of cartilage bounded on either 
side by the developing premaxillze, which are gradually encroaching upon it and causing 
its disappearance. If, however, this encroachment of the premaxille were arrested at 
this stage, the bones of opposite sides remaining separate, instead of fusing across 
the middle line to form the bony spatulate beak of the bird, we should have a 
condition morphologically identical with that in Platypus, due to the persistence of 
the inter-crural prenasal plate of cartilage. But in Platypus we must believe that 
the embryonic prenasal cartilaginous rostrum is a much more extensive structure than 
