146 . EDWARD PHELPS ALLIS, JR. 
PLAGIOSTOMI 
In Chlamydoselachus the lips are much thicker at the angle of 
the gape of the mouth than in their more anterior portions, the 
angle of the gape thus being a relatively long line. The inner 
end of this line forms the functional angle of the gape when the 
mouth is widely opened, and the outer end of the line the func- 
tional angle when the mouth is closed, and from this outer angle 
the outer edge of each lip converges toward the inner edge until 
the lips attain their normal thickness. This is readily seen in 
the accompanying figures 1 and 2, as also in two similar figures 
given by Garman in 1885, and it is there also seen that what is 
actually a portion of the external surface of the head when the 
mouth is widely opened, becomes enclosed between the lips when 
the mouth is closed. 
The cause of this thickening of the lips at the angle of the gape 
was, in the first place, the inevitable formation of a fold in the 
loose dermis at that angle, such a fold being well shown in 
Miiller and Henle’s (1841) figure of Pristis antiquorum, repro- 
duced in the accompanying figure 3, but this slight fold was 
later enlarged, apparently because of the pressure of the thick 
concave anterior edge of the musculus adductor mandibulae, 
where it passed from the upper to the lower jaw, against the 
internal surface of the fold. Because of this pressure, the fold 
was forced outward and forward, and, when the mouth was 
closed, bulged across the primary angle of the gape, its 
anterior surface being presented symphysially and internally 
and added to the lips at the angle of the gape. The crest of this 
fold then formed, when the mouth was closed, a secondary angle 
of the gape, which lay antero-lateral to the primary angle, and 
short secondary upper and lower lips ran forward from it and 
joined the primary lips. That portion of the external surface 
of the head which lay between these secondary lips and the 
primary ones was then first added to the lips at the angle of 
the gape but later incorporated in the buccal cavity by the 
formation of a cheek. The secondary lips, because of the 
manner of their formation, are not at first represented by the 
