LIPS AND NASAL APERTURES IN FISHES E7433 
flap and its related process a would acquire the position of the 
large valvular process of Chimaera. The position of the flap- 
seat and the process 6 would not be affected by this change in 
position of the nasal flap, but the antero-lateral nasal aperture 
would be distorted and would lie somewhat perpendicular to the 
external surface, a passage leading aborally and mesially from it 
to the exterior beneath the overlapping nasal fold. An increased 
development of the nasal fold would then produce the naso- 
labial fold of Chimaera, and the flap-seat (process 8) of Chilo- 
scyllium would become the transverse ridge on the internal 
surface of the fold of Chimaera. The postero-mesial nasal 
aperture would, in the meantime, have been crowded mesially 
beneath the nasal flap and would push it backward, much as it 
is shown artificially pushed back on one side of Miiller and 
Henle’s figure of Chiloscyllium, and the process a, as shown in 
that figure, would swing orally and then. mesially until it came 
in contact with the base of the turned back nasal flap, and a new 
flap-seat would be formed there. 
If the nasal flap, turned back as above assumed, were to fuse 
with that part of the external surface of the snout which lies 
beneath it, the edge of the flap would in part form what I have 
described as the aboral premaxillary lip, and the median incisure 
of that lip would represent the line of incomplete fusion of the flaps 
of opposite sides of the head. The remainder of the edge of the 
flap would encircle a part of the original postero-mesial nasal 
aperture, separating from it a new and smaller aperture, the 
anterior-mesial aperture of the adult Chimaera. The postero- 
lateral nasal aperture of the adult Chimaera would then be 
formed by the naso-buceal groove of Chiloscyllium; that*is, by 
the oral edge of the original postero-mesial aperture together 
with the nasal-flap furrow. This postero-lateral aperture would 
lie internal to the persisting antero-lateral aperture, these two 
apertures having one edge in common, formed by the postero- 
lateral edge of the large valvular process (process a), while the 
other edges of the two apertures would be formed, in the one case 
by the little fold which crosses the floor of the naso-buccal groove 
of Chimaera and in the other by the transverse ridge on the 
