164 EDWARD PHELPS ALLIS, JR. 
to consider the ala nasalis to be cut off from the outer edge of 
the nasal capsule than there is to consider the vertebrae to be 
segmented from the hind end of the cranium. 
The posterior upper labial of Heterodontus francisci projects 
dorsally and somewhat posteriorly from the angle of the gape, 
lies against the postero-lateral surface of the nasal capsule, and 
hence has the position shown in Huxley’s figure of Heterodontus 
Philippi and not that in Gegenbaur’s figure of the same fish 
(72, fig. 3, pl. 12). The anterior upper labial lies antero-mesial 
to and parallel with the posterior one, as shown in Huxley’s 
figure. Thus both these two labials project dorso-posteriorly 
from the angle of the gape, instead of anteriorly, as they do in 
the other Selachii above considered, this apparently being due to 
the fact that the dorso-posterior ends of these labials were attached 
posterior to the nasal capsule, and that accordingly, when their 
ventro-anterior ends were carried forward by the shortening of 
the gape, the labials swung forward around their dorsal points 
of attachment as centers and so became directed dorso-pos- 
teriorly. Because of this position of the labials, there is no 
supralabial furrow. A deep postlabial furrow lies internal to 
the mandibular labial and extends to its anterior end. A small 
supramandibular furrow also occurs, external to the mandibular 
labial and appearing as a groove on the external wall of the 
large postlabial furrow. It is accordingly not seen unless the 
latter furrow be forced open. There is a maxillary preangular 
crease in the upper jaw and a corresponding crease in the lower 
jaw. 
HOLOCEPHALI 
In the Holocephali the lips and the nasal apertures differ 
markedly from those in the Plagiostomi. In Chimaera colliei 
(figs. 18 to 16) the long axis of the fenestra nasalis lies in a plane 
approximately parallel to the lateral edge of the palatoquadrate, 
as it does in Heterodontus, but the line joiming the external 
nasal apertures lies approximately in the same plane, instead 
of, as in Heterodontus, crossing it at a considerable angle. The 
two nasal apertures are accordingly, one postero-lateral, and the 
