BY R. BROOM. 559 



prevomer a small piece of cartilage is seen cut across (-S'.c). This 

 is Stenson's cartilage at the ]3lace where near the posterior part 

 of Jacobson'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 di^ddes 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 ])y 

 Newton Parker t 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 b}^ the great development of the premaxillaries. 

 In the Australian Bat (Miutopferus scJireiberdi , Natt.), however, 

 as the premaxillaries do not meet in the middle line, we have 



* J. T. Wilson. " Observations upou the anatomy and relations of the 

 Dumbbell-shaped Bone in Ornitliorhynrhus, &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 aculeata.''^ Proc. Zool. Soc. 1894. 



