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process and its relations to the orbital process of the palato-quadrate 
might suggest its being the homologue of the basipterygoid process 
of Lepidosteus were it not that there is a much more probable origin 
of the latter process, which will be later explained. 
The acustico-trigemino-facialis recess les in the lateral wall of 
the cranial cavity posterior to the postelinoid wall, and the lining 
membrane of the cranial cavity lines also the walls of the recess. In 
the anterior wall of the recess is the large foramen trigeminum, which 
opens directly into the trigemino-pituitary fossa at the hind end of 
the orbit; in the posterior wall of the recess is the foramen acusticum ; 
and between the two is the internal opening of the canal for the 
nervus facialis. This latter canal is a long one, as is the correspond- 
ing canal in Acanthias vulgaris, and a rather long canal leads from 
it and opens on the antero-dorsal surface of a ledge of cartilage that 
corresponds to the posterior portion only of the large subocular shelf 
of Mustelus, the subocular shelf of selachians developing, according 
to SEWERTZOFF (1899), from independent anterior and posterior 
portions. The canal that opens on the antero-dorsal surface of this 
shelf transmits the ramus palatinus, the long canal, in this fish, replac- 
ing the simple hiatus described by GEGENBAUR in Acanthias vulgaris. 
The nervus abducens, running antero-latero-ventrally, traverses a 
canal in the cartilage and, entermg the pituitary canal, there inner- 
vates the rectus externus. The external carotid artery perforates 
the subocular shelf and issues at the ventral edge of the trigemino- 
pituitary fossa, separated from that fossa by a layer of tough mem- 
brane. The internal jugular vein traverses a large canal, not shown by 
GEGENBAUR, that begins, anteriorly, in the trigemino-pituitary fossa 
and, perforating the basal portion of the postorbital process, opens on the 
lateral surface of the postorbital portion of the chondrocranium slight- 
ly antero-dorsal to the foramen faciale. In Rhynchobatus, Trygon and 
Pristis this internal jugular canal is apparently also found (GEGEN- 
BAUR, 1. c. p. 49), but the posterior opening of the canal has there fused 
with the foramen faciale. GEGENBAUR considered the anterior open- 
ing of this canal, in the three fishes above mentioned, to represent 
the hiatus Falloppü of his descriptions of Acanthias, but this is prob- 
ably an error. The acustico-trigemino-facialis recess of Acanthias 
blainvillii is as GEGENBAUR shows it in Acanthias vulgaris excepting 
that the foramina trigeminum and faciale lie closer together. ‘The 
nervus ophthalmicus profundus issues through the foramen trige- 
