LIPS AND NASAL APERTURES IN FISHES 147 
crest of the fold which gives origin to them, that crest diverging 
more or less from the primary lips and the fold gradually spread- 
ing out upon the external surface of the head and vanishing. 
There is accordingly quite frequently a break in the definitive lip, 
particularly the upper lip, between the primary lips and the 
crest of the fold of the secondary lip; as seen in the accompanying 
figures of Mustelus and Scyllium (figs. 5, 8). 
In all of the few other Plagiostomi that I have been able to 
examine, secondary lips are found which are strictly comparable 
to those above described in Chlamydoselachus, but the short 
secondary lips of the latter fish may be extended much fa. ther 
forward, and there are marked variations in the upper lip due 
mainly to the varying relations of the nasal apertures to the 
upper edge of the mouth. These variations, in the few fishes I 
have been able to examine, will be considered in connection with 
the descriptions of the nasal apertures, but it may here be stated 
that, as a general rule, where the nasal apertures lie at a con- 
siderable distance from the upper edge of the mouth, the 
secondary upper lip passes between those apertures and that 
edge of the mouth, but when the oral nasal aperture lies near the 
upper edge of the mouth, the fold of the secondary upper lip is 
interrupted or displaced by its encounter with that aperture. 
Related to the secondary angle of the gape there are, as is 
well known, in most, but not all of the Plagiostomi, dermal 
furrows, more or less developed. One of these furrows lies in 
the upper jaw, dorsal and internal to the one or two upper labial 
cartilages, and it usually turns downward, posterior and internal 
to the articulating hind ends of those labials with the man- 
dibular labial, and then forward (symphysially) a short distance 
aboral and internal to the latter liabal. A dermal flap, or fold, en- 
closing the articulating hind ends of the upper and lower labials, 
is thus formed, and it may be called the labial fold. The related 
furrow is indistinctly separated into two parts in all the Selachu I 
have examined, these two parts being confluent in their superficial 
portions but slightly separated from each other in their deeper 
portions. They are, however, apparently simply parts of a single 
furrow and can be called, together, the labial furrow; the two 
