LIPS AND NASAL APERTURES IN FISHES 167 
there to the secondary angle of the gape is formed by the oral edge 
of the labial portion of the naso-labial fold, which fold is formed 
by the fusion of a normal labial fold with another, the probable 
origin of which will be considered later. This large naso-labial 
fold completely covers the postero-lateral nasal aperture, the 
naso-buecal groove, and the nasal valvular process, and its 
antero-mesial edge runs antero-dorso-mesially around the lateral 
edge of the antero-mesial nasal aperture and then about half 
way around its dorsal edge. The fold is separated into nasal 
and labial portions by a slight transverse and vertical furrow on 
its external surface, which extends orally from the aboral edge 
of the fold about half way across it. The transverse ridge on 
the internal surface of the fold lies at the posterior end of its 
nasal portion, and hence immediately anterior to the vertical 
furrow referred to above. Anterior to this ridge a smaller trans- 
verse ridge abuts against the external surface of the nasal val- 
vular process. 
A primary lower lip extends the full length of the primary gape 
of the mouth, and a secondary lower lip the full length of the 
secondary gape, each of these lips extending forward to the 
median. line and there being continuous with its fellow of the 
opposite side. 
Aboral to the large naso-labial fold, and aboral also to the 
antero-mesial nasal aperture, is another dermal fold, continuous 
in the median line with its fellow of the opposite side, the united 
ventral edges of the folds of the two sides forming a somewhat 
semicircular line which arches over and frames the nasal apertures. 
and the mouth. This fold can be called, for reasons to be given 
later, the supramaxillary fold, the furrow separating it from the 
underlying tissues being called the supramaxilary furrow. The 
fold, as here developed, is a characteristic feature of all the 
Holocephali that I find figured, and in Chimaera it lodges the outer °. 
ends of a series of ampullary tubules, which open on the external 
surface in a line of ampullary pores immediately dorsal (aboral) 
to the edge of the fold. The latero-sensory canals all lie aboral 
(dorso-posterior) to the fold and hence aboral also to both the 
nasal apertures, the antorbital section of the buccalis latero- 
