BY R. BROOM. 
559 
prevomer a small piece of cartilage is seen cut across (S.c). This 
is Stenson's cartilage at the place where near the posterior part 
of J acobson's organ it passes below the organ and meets its fellow 
of the other side. Behind this are seen the maxillary and the 
vomer, and between this latter and the prevomer the large inter- 
narial opening recently described by Prof. Wilson.* 
In the near ally of the Platypus — the Echidna — the condition 
of the prenasal illustrates the Monotreme type in a much less 
specialised form. Here we have a closer approach to the condi- 
tion in the bird. In fig. 2, PI. xliv., is represented a median 
longitudinal section of the anterior part of the snout. The nasal 
septum (n.s.) closely resembles that in Ornithorhynchus, and like 
it divides into a small upper part {a. n.s.') and a larger lower — the 
prenasal (p.n.). The upper, as in the Platypus, supports the 
alinasals, but instead of passing straight forward, at its anterior 
part it becomes folded back and rests on the symphysis of the 
premaxillaries. In the very young specimens as figured by 
Newton Parker f the prenasal is well developed and bears much 
resemblance to that in Ornithorhynchus. As the animal reaches 
maturity the prenasal, however, becomes reduced by the develop- 
ing premaxillaries into a thin plate of hyaline cartilage lying 
between the adjacent parts of the two bones. In old specimens 
the hyaline cartilage becomes partly ossified and partly converted 
into fibro-cartilage, only a little of the original tissue remaining. 
There is no apparent anterior extension of the prenasal comparable 
to that in Ornithorhynchus. 
As has been already remarked in the higher forms, the prenasal 
is usually aborted by the great development of the premaxillaries. 
In the Australian Bat (Miniopterus schreibersii, Islatt.), however, 
as the premaxillaries do not meet in the middle line, we have 
* J. T. Wilson. " Observations upon the anatomy and relations of the 
Dumbbell-shaped Bone in Ornithorhynchus, &c." Proc. Linn. Soc. N.S.W. 
(2nd Ser.) Vol. ix. 1894. 
t W. N. Parker. "On some points in the Structure of the Young of 
Echidna acideata." Proc. Zool. Soc. 1894. 
