VU SKULL 217 
except where a slender synotic band of cartilage extends 
between the two periotic capsules. The floor is also incom- 
plete, a large pituitary fontanelle remaining to indicate the 
original separation of the trabeculae while transmitting the hypo- 
physial or pituitary caecum. The notochord traverses the floor 
of the parachordal portion of the cranium as far as the pituitary 
fontanelle, and from the sides of the synotic ring the auditory 
capsules project in the shape of conspicuous lateral prominences. 
In front the otherwise open end of the cranial cavity is closed 
by the dorsally-placed and unpaired olfactory capsule, which is 
Loe Lei rice 
brb.s 
SUSU VASE 
olfc wads brbs brb.2 
x La 
‘ . 
Fie. 124.—Skull, with branchial basket and anterior part of the vertebral column, of 
Petromyzon marinus. a.d.c, Anterior dorsal cartilage; a.lat.c, anterior lateral 
cartilage; an.c, annular cartilage ; av.c, auditory capsule ; 67.b.1-9, vertical bars of 
the branchial basket ; br.cl. 1-7, external branchial clefts ; c7.c, cornual cartilage ; 
er.r, cranial roof ; Z.c.1-4, longitudinal bars of branchial basket ; Zg.c, lingual car- 
tilage ; m.v.c, median ventral cartilage ; .a, neural arches ; na@.ap, nasal aperture ; 
n.ch, notochord ; Nv*, foramen for optic nerve; olf.c, olfactory capsule; pe.c, 
cartilage surrounding pericardial cavity ; p.d.c, posterior dorsal cartilage ; p./at.c, 
posterior lateral cartilage ; sb.oc.a, suboeular arch; s¢.p, styloid process ; sty.c, 
styliform cartilage ; ¢, teeth. (From Parker and Haswell, after W. K. Parker.) 
perforated behind by two apertures for the olfactory nerves, and 
has only a fibrous connexion with the cranial walls. Anteriorly 
to the olfactory capsule the cranial floor is prolonged forwards 
over the roof of the mouth as a large laterally-expanded plate, 
formed by the united anterior portions of the trabeculae, and no 
doubt representing the mesethmoid cartilage of the Dog-Fish. So 
far the cranium presents no special difficulty, and in its general 
features may be readily compared with that of an embryonic 
Elasmobranch. As for the rest of the skull, it is obvious that it 
has been greatly modified, partly to form and to support the 
skeletal framework of the remarkable suctorial buccal funnel, and 
partly to form the singular rasping lingual apparatus. Hence it 
is always difficult and sometimes impossible to identify with 
