OF THE SKULL IN SHARKS AND SKATES. 207 
plex cartilaginous box, as complete as the bony box seen in the skull of the higher 
kinds of birds. Here, if anywhere, the cranial segments ought to be found. But, mor- 
phologically, these types are not at a very low level; certainly, if the cranium, even 
behind the pituitary body, be the result of slow secular consolidation of a series of 
vertebre, these fishes would seem to be a very late product of evolution. 
The truth of the matter is, that the Sharks and Rays are very enigmatical as to their 
position in the vertebrate series; below the Teleosteans as to their skull, by two 
important steps or degrees (as was shown in the paper on the Salmon’s skull), they yet 
come much closer to the tail-bearing and tailless Amphibia than any other fishes with 
the exception of the “ Dipnoi.” 
These things must be borne in mind whilst studying the conditions of the adult 
skull of a type which undergoes no bony metamorphosis, yet has so very perfect a 
cranium and a large and perfect basket-work of visceral arches. 
The cranium itself (Pl. XX XVII. figs. 2,3, Pl. XX XVIII. figs. 1, 2) is a very elegant 
structure; it is a flat-bottomed barge, like that of the frog (‘ Frog’s Skull,” pl. 9. 
figs. 6 & 7), but having a cartilaginous deck. On each side, in front, there is an elegant 
dome-shaped “ awning "—the roof of the nasal sac; the deck, or tegmen cranii, is wide 
open in front; beyond this opening a small ‘“ prow” projects, the “prenasal or basi- 
trabecular” cartilage ; and this is spliced obliquely by a pair of bars, the foremost extra- 
viscerals, which were in front, simply, and now overlie the nasal roofs (Pl. XX XVII. 
fig. 2, 7. 1). 
The basal view (Pl. XX XVII. fig. 3) shows the almost uniform breadth of the whole 
of the occipital and sphenoidal regions, the centre of the post-sphenoidal territory 
being shown by the entrance of the internal carotid (7.c). The occipital condyles 
(0c.c) project but little; the ridges of the otic capsule square the skull behind; and 
these ridges form a “ tegmen tympani,” under which there is no tympanic cavity, but a 
condyloid subconcave facet for the huge representative of the incus, the “ epihyal ” or 
hyomandibular. The lower edge of that facet is formed by the investing mass (i v) ; 
between this and the flat, outspread trabecular elbow (p.tr), is a notch; and a lesser 
notch separates the elbow from the rest of the trabecular plate, which further forwards 
narrows again, and then sends out the antorbital spur(a.0). The trabecule then suddenly 
contract, and grow upwards (Pl. XX XVII. fig. 3); and their ascending part becomes now 
coalesced with the inner edges of the nasal-roof cartilages, thus forming a primordial 
mesethmoid or septum between the nasal sacs, which is normally composed of four carti- 
laginous growths. On each side of this middle wall a membranous space, open in the 
middle, forms a sort of trap-door down into the nasal sacs, through which the olfactory 
fibres pass to the nasal plice, which are pinnately arranged, and entirely membranous. 
These spaces answer to the moieties of the cribriform plate of the mammal. The 
second labial (/. 2) has partly coalesced with the anterior edges of the nasal dome, and 
with the corresponding cornu trabecule (c.f). The third labial (/. 3) is precisely like 
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