354 EDWARD PHELPS ALLIS, JR. 
a 114-mm. specimen (l.c., fig. 6, pl. 31), this pit-like depression 
has become a long slit-like furrow which extends posteriorly 
beyond the angle of the gape and there vanishes on the outer 
surface of the cheek. The posterior portion of the upper lip, 
which in the 10-mm. specimen was nearly straight, has, in the 
114-mm. one, grown ventrally so that it overlaps externally the 
dorsal edge of the mandible. This overlapping portion of the 
lip forms the ventral half of the future labial fold, and its in- 
ternal surface is formed by part of the internal surface of the 
secondary lip. The maxillary preangular crease of the Selachii 
should accordingly, if present, be found on this surface. In the 
lower lip a furrow has been developed which, for reasons to be 
later given, would seem to be a labial-flap furrow and not a 
sublabial one. 
In later stages the hind edge of that part of the upper lip that 
overlaps externally the dorsal edge of the mandible is prolonged 
posteriorly, and becomes separated from the side of the head by 
a crease, rather than furrow, which extends a short distance 
posterior to the angle of the gape and corresponds to the sub- 
maxillary furrow of Acanthias. A postlabial furrow is then 
formed, this furrow extending downward from the supralabial 
furrow to the hind end of the submaxillary crease. The hind 
end of the entire labial fold then grows posteriorly, and the fold 
is completely differentiated, this apparently taking place at 
different ages in different specimens, for the completed fold is 
shown in my figure of an 18-mm. specimen, but not yet com- 
pletely differentiated in a 3l-mm. one (l.c., figs. 12 and 13, 
pl. 34). In these older specimens the supralabial furrow extends 
anteriorly to the anterior end of the lachrymal bone, and there 
ends immediately posterior to the articular head of the maxillary. 
It is, however, the relation to the lachrymal, and not that to the 
articular head of the maxillary, that determines the length of 
the furrow, for when the lachrymal has an anterior extension, as 
in Gadus, the furrow continues to its anterior end. 
On the roof of the mouth (figs. 15 to 18), immediately external 
to, and concentric with, the pterygo-palato-vomerine teeth, there 
is a sulcus which is the homologue of the primary superior 
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