226 
thias blainvilli. Comparison will then be made with Polyodon, 
Lepidosteus, Amia, Scomber and the Mail-Cheeked fishes. 
The cavum cranii of the adult Chlamydoselachus is closed an- 
teriorly by the thick membrane that separates it from the cavum 
praecerebrale (Auıs, 1913). Immediately posterior to this membrane, 
on either side, the lateral wall of the cavity is perforated by the large 
but short canalis olfactorius which runs antero-laterally and opens 
into the dorsal portion of the cavum nasi. 
Posterior and slightly ventral to the internal opening of the 
canalis olfactorius is the internal opening of the foramen opticum, 
and in a line passing along the hind edge of this foramen, and inclining 
slightly from below upward and backward, the cranial cavity is some- 
what constricted by a thickening of its dorsal and ventral walls. 
Dorsally this thickening underlies, or lies slightly anterior to, what 
is in certain specimens a small median depression (GooprEy, 1910, 
Fig. 2) and in others a small median gash which does not extend 
through the cartilage (GARMAN, 1885, Pl. 8). Ventrally, the thickening 
forms the presphenoid process (Vorsprung) of GEGENBAUR’s (1872) 
descriptions of other selachians, this process being, in Chlamydo- 
selachus, a transverse bolster which lies beneath the optic chiasma and 
either directly dorsal to, or slightly anterior to, a marked angle in the 
mid-ventral line of the neurocranium. This angle lies directly be- 
tween the articular surfaces for the orbital processes of the palato- 
quadrates, and I have already referred to it, in two of my earlier 
works (1913 and 1914), as the basal corner (Basalecke, GEGENBAUR). 
According to SEWERTZOFF (1899) the basal corner of the se- 
lachian chondrocranium represents the point where the trabeculae, 
primarily laid down perpendicular to the ventral surface of the para- 
chordals, bend somewhat abruptly forward to assume a more or less 
horizontal position in the lme prolonged of the parachordals, and 
SEWERTZOFF evidently considers that the position of this bend, 
which he identifies as the basal corner of GEGENBAUR’S descriptions, 
is determined by the point of attachment, in embryos, of the orbital 
process of the palato-quadrate of either side with the corresponding 
trabecula (l. c. pp. 296 and 298). And the basal corner of the adult 
selachian, as first described by GEGENBAUR, was, in fact, said by that 
author to lie between the articulations of those two orbital (palato- 
basal) processes, one on either side of the head, with the orbital walls. 
But in Sewerrzorr’s figure of a 50 mm. embryo of Acanthias, and still 
