or THE SKTJLL 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 

 vertebrte, these fishes would seem to be a very late product of evolution. 



The truth of the matter is, that the Sharks and Eays 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 (PI. XXXVII. figs. 2, 3, PI. XXXVIII. figs. 1, 2) is a very elegant 

 structure ; it is a flat-bottomed barge, like that of the frog (" Frog's Skull," pi. 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 "prsenasal 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 XXXVII. 

 fig. 2, I. 1). 



The basal view (PI. XXXVII. 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 {i.c). The occipital condyles 

 {oc.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 (? 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.o). The trabeculse then suddenly 

 contract, and grow upwards (PL XXXVII. 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 plicce, which are pinnately arranged, and entirely membranous. 

 These spaces answer to the moieties of the cribriform plate of the mammal. The 

 second labial (I. 2) has partly coalesced with the anterior edges of the nasal dome, and 

 with the corresponding cornu trabeculge (c.tr). The third labial (I. 3) is precisely like 



2g2 



