OP THE SKULL IN SHAEKS AND SKATES. 203 



as thickenings over the eye (PI. XXXIV. figs. 1 & 2, s.ob, and also in the Salmon, 

 "Salmon's Skull," pi. 1. figs. 1, 3, 6 & 7). We thus come to a proper understanding 

 of the nature of the ecto-ethmoidal masses of the higher vertebrata ; they are composed 

 of the proper olfactory domes, and the superorbital bands, which reappear in front 

 beneath the nasals and frontals at their contiguous edges (" Fowl's Skull," pi. 83. 

 figs. 2 & 5, al.e). The extreme simplicity of these primordial olfactory capsules (they 

 send no outgrowths between the folds of the Schneiderian membrane) makes them of 

 great use for unlocking the difficulties in the higher types, where several regions are 

 specialized and many outgrowths formed. 



In the Shark there is no " fontanelle " proper ; but the roof of the cranium (" tegmen 

 cranii ") ceases in front, close to the nasal sacs (PL XXXVI. fig. 3, t.cr, na), so that the 

 cranial cavity is open in front, and the long rhimencephalic crura diverge and pass into 

 the nasal region beneath the inner side of the dome (fig. 4). These nervous masses lie 

 on the trabeculae, and escape into the nasal sac through a space formed between the inter- 

 nasal region of the trabeculae and their olfactory cornua {c.tr); this "fenestra" answers 

 to the chink in the Bird and to the cribriform plate in the Mammal. The diverging 

 crests of the trabecular cornua (figs. 3, 4) being so outspread, the " olfactory fenestrse " 

 are in this stage nearly horizontal ; but in the adult (PI. XXXVII. fig. 3. I) they are 

 slanting, their direction being upwards and forwards from the narrowed trabecular bar 

 to the postero-superior margin of the olfactory dome inside. Looking at the beaked 

 face of a large shark, such as the Porbeagle [Lamna cornubica), it does not at first seem 

 evident what the two clumsy-looking rods of cartilage are that converge towards the end 

 of the basitrabecular rostrum. In the present subject {Scyllium) they are flat bands 

 burrowed by slime glands (PI. XXXVII. fig. 3, U). In this early stage they are easily 

 interpreted (PI. XXXVI. figs. 3, 4, 5, and PL XXXVII. fig. 1, l\ P, P) ; they are two 

 of the labial (" extravisceral "} cartilages that cluster round the large nasal opening, to 

 which they are related as valvular folds. 



The second and third of these are applied closely to the edges of the dome. The 

 second is in front ; and this is of a crescentic shape ; it enlarges the nasal cavity, and 

 partly floors it. The third is a thick ear-shaped cartilage on the outer edge of the 

 nasal dome ; this is the " appendix alse nasi " of human anatomy. 



The first of these cartilages is in front of the second ; it is somewhat heart-shaped, 

 and slightly overlaps the second ; already it is the recipient of the two or three slime- 

 glands which have burrowed its upper surface (PL XXXVI. fig. 3, l^). Afterwards it 

 reaches over the nasal dome, and contains many glands (PL XXXVII. fig. 3, l^). The 

 fourth labial (PL XXXVI. fig. 4, l^) is lanceolate with its stout end forwards ; it under- 

 lies the quadrato-pterygoid bar [q.pcj), and lies on the upper edge of the angle of the 

 mouth. A similar cartilage, converging towards the last, lies on the lower edge of the 

 ano-le of the mouth, attached to the infero-lateral surface of the mandible. This is 

 the fifth labial (PL XXXVL fig. 4, P). Similar cartilages, " extrabranchials " (see 



