LIPS AND NASAL APERTURES IN FISHES 179 
becomes directed postero-laterally. The anterior end of the 
groove in the younger embryos shown by these authors apparently 
corresponds to the same end of the groove in the older ones, the 
aboral (anterior) nasal aperture of the adult fish (the mceurrent 
aperture), accordingly corresponding to the antero-lateral and 
also incurrent aperture of the Selachii. The line of the nasal 
groove shown in the older embryos, which coincides in direction 
with the line of the nasal apertures of the adult, accordingly 
rotates in this fish in the opposite direction to that in which it 
rotates in the Selachii and Holocephali. In the Teleostei the 
line of the nasal apertures also rotates slightly in this same 
direction, as already stated, and this is also the case in the 
Amniota. There must then be some reason for this difference 
in the direction of rotation of this line in these different verte- 
brates, and it would seem to be related to the position of the 
septum nasi, for this septum, when present in the Plagiostomi 
and Holocephali, lies ventral to the trabeculae, while in Cerato- 
dus and in the Teleostomi and Amniota it lies dorsal to the 
trabeculae. . 
Ginther (71) says that both the nasal apertures of the adult 
Ceratodus lie within the cavity of the mouth. Huxley (’76) 
later concluded that ‘‘the anterior nares can in no sense be said 
to open into the cavity of the mouth, inasmuch as they lie outside 
the premaxillary portion of the upper lip, and are not enclosed 
by the maxillary portion of that lip. They are not even placed 
between the upper and the lower lips, inasmuch as the vaulted 
flap, on the under side of which they lie, is not the upper lip, but 
the anterior part of the head.’’ Semon (’93) says that, in em- 
bryos of this fish, the anterior aperture lies anterior to the upper 
edge of the mouth and the posterior aperture posterior to that 
edge, and that the two apertures arise through the coalescence 
of the opposite lips of a naso-buccal groove. Gegenbaur (’98) 
says that the posterior aperture, alone, is a choana, the anterior 
aperture lying on the edge of the lip, and hence not in the cavity 
of the mouth, and representing the primitive opening of the 
nasal pit; but he was uncertain as to whether or not the posterior 
aperture was derived, as he says it is in certain of the Selachii 
