OF THE SKTILL IN SHAEKS AND SKATES. 209 



anterior canal is seen a short thick process, the sphenotic process (s/>.c) : it is formed by 

 the grafting of the superorbital arc (s.ob) on to the auditory mass. Behind the notch is 

 the large " pterotic" eminence (2}to), 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. 



Aftei-wards, 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 XXXVII. fig. 4) shows how the brain (CI, 

 Cla, C2,Co) 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 (^.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 fi-ont 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 trabeculse ; 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 PI. 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 tesserae, 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 number this and many other sharks agree 

 with the Osseous Fishes, there being seven postoial arches ; there are more in Hexan- 

 chus and Heptanchus, which have respectively six and seven persistent branchial slits, 

 besides the " spiracle." 



