158 EDWARD PHELPS ALLIS, JR. 
formed. The conditions in the two fishes are so strictly similar 
that it seems to me there can be no possible doubt of this, and 
the formation of the groove in Raia is related, as it is in Scyllium, 
to a nasal-flap furrow which extends to the upper edge of the 
mouth. The fold of the secondary upper lip does not however, 
in Raia, abut against the naso-buccal groove and end there, as it 
does in Scyllium, for it has been deflected from its forward 
course by the coalescence, with the naso-buccal groove, of the 
crease between the secondary and primary upper lips. The 
crest of the fold of this lip accordingly retains its normal relations 
to this crease and runs aborally along the lateral edge of the 
groove. In those of the Raiidae in which the nasal-flap furrow 
does not extend to the upper edge of the mouth there is no 
naso-buceal groove, and the secondary upper lip runs forward, 
oral to both nasal apertures, exactly as it does in those Selachii 
in which this groove is not found. 
In Heterodontus (probably francisci), a single specimen of 
which I have had at my disposal (figs. 6 and 7), the outer end of 
the line of the angle of the gape lies at a relatively considerable 
distance from the lateral edge of the palatoquadrate, and it has 
been carried forward into the transverse plane of the hind end 
of the nasal capsule, or even slightly anterior to that plane. 
Because of this shortening of the length of the gape without a 
corresponding shortening of the line of the angle of the gape, the 
fold of the secondary upper lip is tall, and in running forward, 
it immediately reaches the process 6 and is there directed 
between the two nasal apertures. On the mesial side of the 
nasal apertures the line of this secondary fold is continued by 
a well developed fronto-nasal flap, or so-called process. A well 
developed primary upper lip runs forward along the external 
edge of the palatoquadrate dental arcade until it has passed 
the postero-mesial nasal aperture, where the fold spreads out 
on the internal surface of the fronto-nasal flap and vanishes as a 
distinct fold. The edge of the fronto-nasal flap is certainly not 
formed, in any part, by this lip, and it must accordingly either 
represent an anterior continuation of the crest of the fold of the 
secondary upper lip, or be a special and independent formation. 
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