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more in GEGENBAUR’s figure (I. c. Pl. 4, Fig. 2) of the adult Hexanchus, 
there are two bends or angles, instead of only one, in the mid-ventral 
line of the chondrocranium: the one lying immediately anterior to 
the fenestra hypophyseos, or its topographical equivalent the foramen 
or canal by which the internal carotid artery perforates the basis 
cranii, and the other lymg approximately beneath the optic chiasma. 
Hither one or the other, or even both, of these two angles may be 
wanting in the adult selachian. In Hexanchus, for instance, both 
angles are shown by GEGENBAUR, and the articulation of the orbital 
process is in the region of the anterior one; in Squatina (Rhina) the 
anterior angle alone is shown, and the orbital process is in articular 
relations with it; in Acanthias the posterior angle alone is shown, 
and the orbital process is in articular relations with it; while in Cen- 
trophorus there is but one angle, the orbital process is in articular 
relations with it, and it lies markedly between the carotid canal 
and the presphenoid bolster. In Chlamydoselachus, as can be seen 
in GoopEy’s figures, the basal corner, identified by its relations to 
the orbital processes, lies in the region of the presphenoid bolster, 
and I find, in all my specimens of this fish, a second and well marked 
angle in the mid-ventral line, not shown by Goopery, that lies either 
directly between or but slightly anterior to the foramina carotica. 
It is thus evident that the articulation of the orbital process of 
the palato-quadrate with the cranial wall is not, in the adult selachian, 
a topographically fixed point, and that the so-called basal corner may 
be found anywhere in the region included between the presphenoid 
bolster and the mternal carotid foramen. And it seems further evi- 
dent that the point where the trabeculae, in embryos, first bend for- 
ward from a vertical to a nearly horizontal position always les in 
the region of the internal carotid foramen of the adult, that foramen 
marking the position of the fenestra hypophyseos of embryos, 
and that the position of this bend is primarily determimed by the 
overlying brain and not by the points of attachment of the orbital 
processes of the palato-quadrates. 
Posterior to the presphenoid bolster, in Chlamydoselachus, and 
extending to the postelinoid wall (Sattellehne, GEGENBAUR) there 
is, in the prepared skull, a large basin-like depression in the cranial 
floor, this depression being the Sattelgrube of GEGENBAUR'S des- 
criptions of other selachians and, as in those selachians, lodging the 
lobi inferiores, the hypophysis and the saccus vasculosus. In the 
15% 
