OF THE SKULL IN SHARKS AND SKATES. 209 
anterior canal is seen a short thick process, the sphenotic process (sp.c): it is formed by 
the grafting of the superorbital arc (s.0d) on to the auditory mass. Behind the notch is 
the large “ pterotic” eminence ( pto), containing the horizontal canal (h.s.c). Whatever 
the superorbital arc may be morphologically, it is a structure of the greatest importance ; 
here, at its fullest growth, it shows that the “lateral ethmoid” and the periotic capsule 
are made compound by coalescence with it ; and this original composition of these parts 
must never be forgotten in an ascending survey. 
Thus the nasal capsules are mixed up, or confluent, with the trabecular and basi- 
trabecular bars, with the superorbital arcs, the “tegmen cranii,” and one pair at least 
of the labials. The periotic capsule is fused with the “ parachordal” bands, or investing 
mass, with the arch growing upwards from that mass (occipital arch), with the tegmen 
cranii, and with the superorbital arc. 
Afterwards, when we come to study such skulls as have the chondrocranium ossified 
into certain definite (interneural) bony territories, we shall often see a single bone 
formed in what was a very complex part originally; and therefore such bones must be 
considered as the products of metamorphosis, and not primordial elements of the 
cranium. 
A section taken longitudinally (Plate XX XVII. fig. 4) shows how the brain (C1, 
Cla, C2, C3) fills the cranium, and how the occipital ring and tegmen cranii cover the 
greater part of the brain. Below, the notochord only persists between the atlas and the 
basis cranii; further forwards the internal carotid (i.c) is seen entering the skull-base 
below the sella turcica (see also fig. 5), which has an anterior and a posterior clinoid 
wall (a.cl, p.cl) and contains the tear-shaped pituitary body, above and in front of which 
is the hollow infundibulum (inf). 
In front the mesethmoidal fenestra (tr.f) is seen to be a mere membranous space 
where the nasal sacs have not thoroughly coalesced with the trabecule; in birds this 
fenestra is formed by the reopening of the cartilaginous wall after complete fusion has 
taken place. 
When the brain has been removed, the various openings can be seen for the exit 
of the nerves: these lie low down; and the chiefest of them can be easily determined 
(see Pl. XXXVIII. fig. 4. 1, 2, 5, 7, 8,9, 10). That for the olfactory (1) is a large 
obliquely tilted window—the membranous “ cribriform plate.” 
The chondrocranium is not more massive and complete than the facial or visceral 
arches, which here attain their utmost size, and undergo no further histological meta- 
mophosis than the calcification, in tessere, of the superficial cells of the hyaline car- 
tilage. Arrest in metamorphosis (as to bony deposit) is here the correlate of large 
development of the parts as to size; in mwmber this and many other sharks agree 
with the Osseous Fishes, there being seven postoral arches; there are more in Hevan- 
chus and Heptanchus, which have respectively six and seven persistent branchial slits, 
besides the “ spiracle.” 
