74 
ZOOLOGY: E. P. ALUS 
addition to it. Successive stages in this process are actually found 
persisting in the adults of living Plagiostomi. 
The fold of the secondary upper lip of either side, running forward, 
passes either (l) between the primary upper edge of the mouth and the 
oral one of the two nasal apertures of its side, (2) across that aperture, (3) 
or between it and the aboral aperture, this depending upon the position 
of the nasal apertures relative to the upper edge of the mouth, upon the 
height of the fold of the secondary upper lip, and upon the length of the 
secondary gape of the mouth. 
In all of the Teleostomi that I have been able to examine, the fold 
of the secondary upper lip passes oral to both the nasal apertures, as it 
also does in all of the Plagiostomi that I have been able to examine in 
which there is no naso-buccal groove, excepting only Heterodontus. 
Where there is a naso-buccal groove, the fold either (l) abuts against that 
groove and there abruptly ends (Scyllium canicula), (2) forms the lateral 
edge of that groove (Raia clavata) , (3) or, as shown in Miiller and Henle's^ 
figures of Ginglymostoma concolor and Stegostoma fasciatum, passes 
between the nasal apertures and is continued mesial to them- as a slight 
fron to-nasal ridge. The naso-buccal groove is simply the oral edge of 
the oral nasal aperture combined with the nasal -flap furrow, and 
results from the obstruction, by that aperture and the nasal flap, of 
the normal progression of the fold of the secondary upper lip. In 
Heterodontus francisci, where this groove is not found, the fold of 
the secondary upper lip passes between the two nasal apertures and 
is continued mesial to them as a well developed fronto-nasal flap, or 
process, and the posterior nasal aperture, lying oral to the fold of this 
lip, is enclosed in the secondary cavity of the mouth; but it still 
lies external to the primary cavity, exactly as the corresponding 
aperture does in all of the other Plagiostomi examined. In the Holo- 
cephali the conditions are too complicated to be here explained, but 
they have been derived directly from those in those of the Plagios- 
tomi in which there is a naso-buccal groove. In the Dipneusti the 
fold of the secondary upper lip passes either between the two aper- 
tures or oral to both of them, the conditions in the one specimen of 
these fishes that I have been able to examine (Ceratodus) not giving 
definite indications as to this. 
The so-called fronto-nasal process of those Plagiostomi in which 
there is a naso-buccal groove and in which the oral edge of the nasal 
flap is extended orally so as to bound the upper edge of the mouth, as 
in Scyllium canicula, is not the homologue of the fronto-nasal process 
of Heterodontus, for the nasal flap arises wholly independently of the 
