170 EDWARD PHELPS ALLIS, JR. 
lip may be ealled the supplementary, and the latter the actual 
secondary lower lip. 
When the mouth is closed, the supplementary lower lip, as 
above defined, abuts against the posterior surface of the larger 
of the two transverse ridges on the internal surface of the naso- 
labial fold, this posterior surface, abutting as it does against 
the supplementary secondary lower lip, thus being a supple-. 
mentary secondary upper lip. The corresponding portion of 
the actual secondary upper lip is formed, as already stated, 
by the ventral edge of the labial portion of the. naso-labial fold, 
and it extends forward from the secondary angle of the gape to 
the vertical furrow which separates this portion of the fold from 
the nasal portion. Anterior to this vertical furrow the line of 
the secondary upper lip is continued onward along the rounded 
ventral edge of the nasal portion of the naso-labial fold, but this 
edge, although, like the oral edge of the nasal flap in Seyllium 
and Raia, it forms part of the upper edge of the mouth, is no 
morphological part of the fold of the secondary upper lip. The 
vertical furrow cn the external surface of the naso-labial fold 
lies directly external to the line between the short supplementary 
secondary upper and lower lips, and has evidently been retained, 
though, as will be later explained, probably not caused, by the 
tissues of the thin labial fold there being creased by falling 
slightly in between the two lips. 
The supplementary secondary upper and lower lips form the 
bounding side wall of the buccal cavity when the mouth is closed, 
a supplementary gape of the mouth, which lies between the 
primary and secondary gapes, thus being formed. The line of 
this supplementary gape runs dorso-posteriorly at a considerable 
angle to the line of the gape of the jaws, its inner end turning 
dorsally and but slightly posteriorly, and the mandibular and 
posterior upper labials articulate with. each other immediately 
dorsal (morphologically posterior) to the inner end of the line. 
These labials thus lie not far from the inner end of the line of the 
angle of the gape, instead of near the outer end, as in the 
Plagiostomi, and they lie, when the mouth is closed, external to 
the palatoquadrate. The mandibular labial has, in consequence, 
