LIPS AND NASAL APERTURES IN FISHES 159 
Its position, definitely in the line of the fold of the secondary 
upper lip, is strongly in favor of its being an anterior continua- 
tion of that fold, and such I consider it to be, since it is not 
found in any fish I know of in which the fold of the secondary 
upper lip has not been interrupted by meeting some part of the 
nasal groove. The fold of the secondary lower lip is continued 
forward approximately to the level of the anterior end of the 
distinctly evident portion of the primary upper lip, and from 
there onward the lower lip of the fish is a primary one. 
The fenestra nasalis of this fish is long and narrow, and’ its 
long axis is directed from in front postero-laterally, approximately 
parallel to the secondary upper lip and hence diverging laterally 
both from the primary upper lip and the lateral edge of the 
palatoquadrate. The incurrent nasal aperture lies aboral to the 
line of the secondary upper lip, the excurrent aperture oral to 
that line; and the planes of the two apertures are inclined to 
each other to such an extent that the internal angle between the 
two planes is less than a right angle. The two apertures are in- 
completely separated from each other, as in the other Selachii, 
by an incompletely formed nasal bridge, which lies in the line 
of the crest of the fold of the secondary upper lip. The excurrent 
aperture is surrounded by a frill of dermal tissues, apparently a 
modification of the nasal flap, the frill being continued, as a 
fold, partly around the incurrent aperture. The nasal section 
of the buccalis latero-sensory canal passes oral to this frill, 
between it and the lateral edge of the palatoquadrate, thus 
encircling the oral edge of the excurrent aperture, as it does in all 
other Plagiostomi. 
The excurrent nasal aperture is thus enclosed within the 
buccal cavity when the mouth is closed, but it lies definitely 
between the secondary and primary upper lips, aboral to the 
latter, and hence in the same relation to it, to the palatoquadrate 
dental arcade, to the buccalis latero-sensory canal, and to the 
incurrent aperture which the corresponding aperture has in the 
other Plagiostomi considered above. There is accordingly no 
possible doubt that the excurrent aperture of Heterodontus is 
the strict homologue of the corresponding aperture in other 
