168 EDWARD PHELPS ALLIS, JR. 
sensory canal not passing, as it does in all of the Plagiostomi and 
Teleostomi, between the upper edge of the mouth and the nasal 
apertures. 
The supramaxillary furrow is always deepest in its posterior 
portion, diminishing anteriorly to a shallow suleus, and it varies 
in depth in different specimens, this apparently being wholly 
due to a greater or less oral extension of the outer edge of the 
fold, the fold in some specimens only overlapping the aboral edge 
of the labial fold while in others it extends beyond the middle 
line of that fold. The furrow extends dorsally into the tissues of 
the head, the side walls of the furrow lying parallel to the ex- 
ternal surface of the head, and the fold being, in consequence, a 
thin sheet of integumental tissues. 
A postlabial furrow occurs in normal position, internal to the 
hind end of the labial fold, the end of the fold enclosing the 
ventral end of'a labial cartilage which I consider, with Vetter 
(78), to be the mandibular labial, instead of enclosing, as in the 
Selachii, the articulating hind ends of that labial and one or both 
of the upper labials. Starting from below and running upward, 
the bottom of the postlabial furrow crosses the internal surface of 
the mandibular labial, and, on reaching its dorso-posterior edge, 
closely approaches the bottom of the supramaxillary furrow, but 
it never, in my specimens, falls directly into that furrow. The 
outer portions of the two furrows are however here confluent. 
The postlabial furrow then turns anteriorly and, crossing the 
external surface of the mandibular labial, reaches a point imme- 
diately ventral (symphysial) to the line of articulation of the 
mandibular and posterior upper labials, the latter labial being 
‘the maxillary labial of Vetter’s descriptions. There the furrow 
falls, at nearly a right angle, into the vertical furrow already 
referred to as separating the large naso-labial fold into nasal and 
labial portions. This vertical furrow lies in the direction pro- 
longed of the line of articulation of the mandibular and posterior 
upper labials, and directly external to the line between the sup- 
plementary secondary upper and lower lips, to be described later. 
It extends about half way across the naso-labial fold, and has no 
homologue in the Selachii. 
