LIPS AND NASAL APERTURES IN FISHES _ 149 
so-called, in this fish. In Mustelus (probably vulgaris) and 
Triakis fasciatum I find all the furrows well developed, and they 
are shown in the accompanying figures of Mustelus (fig. 8). In 
Seyllium canicula (fig. 5) I find the supramandibular furrow and 
the two preangular creases well developed, but there is no post- 
labial or supralabial furrow. In Raia clavata none of these 
furrows are found as such, but the naso-buccal groove has . 
probably absorbed the maxillary preangular crease, as will be 
later explained. 
In the adults of all of the Plagiostomi the primitive single ex- 
ternal opening of the nasal pit is more or less completely, but 
never completely, separated into two parts either by the well 
known nasal flap, which projects from one side of the primitive 
nasal opening and rests upon a flap seat on the other side, or by 
the nasal flap and seat together with two deeper-lying flaps, one on 
either side, which together form what I shall eall the nasal valve 
and its valve seat. The nasal flap and nasal valve of one side 
of the nasal pit, and the flap-seat and valve-seat of the other, 
correspond to the two halves of the well known nasal bridge of 
the Teleostei, but these two halves of the bridge never fuse 
with each other in the Plagiostomi, the two nasal apertures 
never, In consequence, being completely separated from each 
other. ; 
The two nasal apertures le, in the adults of all of the Plagi- 
ostomi that I have examined or can find described, one lateral 
or antero-lateral to the other, and it is always the lateral one 
of the two which serves for the ingress of the current of water 
passing through the nasal pit and the other for its egress. The 
nasal groove of embryos of these fishes, as shown in figures, 
always runs from in front orally and mesially, as does the line 
of the external nasal apertures of the adult, but the line of the 
groove is always inclined to the axis of the body at a smaller 
angle than the line of the apertures. The line of the median 
raphe of the Schneiderian membrane, lies, in the adult, ap- 
proximately in the plane of the long axis of the fenestra nasalis, 
_and crosses the line of the external apertures at a variable 
angle, running, where the mouth is ventral and in the few speci- 
