BY R. BROOM. 553 



trabecular cornu, and any one who had only studied it in the 

 rudimentary condition in which it is found in the higher mammals 

 might readily come to that conclusion. The posterior attachment 

 of the cartilage, however, in the lizard and marsupial proves, I 

 believe, that the paraseptal is a true portion of the nasal capsule. 



Whatever, however, be the morphological significance of the 

 paraseptal cartilage there is no doubt that it is in connection 

 with this cartilage that the so-called "vomer "of the lizard is 

 formed. On transverse section the cartilage is seen passing 

 downward and a little outwards from near the base of the septum, 

 and it is as a splint on the inner side of this cartilage that the 

 " vomer " first develops. It soon, however, passes outwards 

 round the lower border of the cartilage, and forms to some extent 

 a floor to the nasal capsule; but though it extends laterally 

 beyond the limits of the cartilage its length is determined b}^ the 

 length of the paraseptal. 



In Ornithorhynchus and Minioi^teriis the paraseptals, being 

 retained, as in most mammals, only as supports to Jacobson's 

 organ, have their length determined by the length of the organ, 

 and are hence much shorter than the nasal capsule. On the 

 inner side of each cartilage a splint bone develops exactly as in 

 the lizard, and, as in the lizard, the length of these splints is 

 determined by the length of the cartilages. In both Ornitho- 

 rhynchus and Miniopterus these splints in their later development 

 become anchylosed, in the one case to form the dumbbell bone, 

 and in the other the little median bone which lies in front of the 

 vomer. 



As the paraseptal cartilage in the marsupial is undoubtedly 

 the true homologue of the paraseptal cartilage in the lizard, the 

 paraseptal in Ornithorhynchus, even though somewhat rudimen- 

 tary, must also be homologous. It is, therefore, impossible to 

 doubt that the splint bones which form on their inner sides, and 

 whose lengths are determined by the cartilages, must also be 

 homologous. As the so-called " vomer " of the lizard is thus seen 

 to be the homologue of the dumbbell bone of Ornithorhynchus, it 

 can no longer be regarded as a true vomer, since the dumbbell 



