. The elasmobranch fishes . Fig. 211. Dorsal view of brain. (From Locy.) A. Scoliodon. B. Haja. cb., cerebellum; , restiform body; di., diencephalon; , median olfactory nucleus; med., medulla; np., neuropore; , olfactory bulb; , olfactory lobe; , olfactory tract; , optic lobe; tJ., telencephalon; /«., terminal nerve; I, olfactory nerve; II, optic nerve; IV, trochlearis or fourth nerve ; X, vagus nerve. pair of rounded masses located terminally between the olfactory tracts (, fig. 213) and extending ventrally. Much the same condition obtains in Squalus sucklii (se
. The elasmobranch fishes . Fig. 211. Dorsal view of brain. (From Locy.) A. Scoliodon. B. Haja. cb., cerebellum; , restiform body; di., diencephalon; , median olfactory nucleus; med., medulla; np., neuropore; , olfactory bulb; , olfactory lobe; , olfactory tract; , optic lobe; tJ., telencephalon; /«., terminal nerve; I, olfactory nerve; II, optic nerve; IV, trochlearis or fourth nerve ; X, vagus nerve. pair of rounded masses located terminally between the olfactory tracts (, fig. 213) and extending ventrally. Much the same condition obtains in Squalus sucklii (see p. 183, fig. 172a), but in this form the median nucleus is not pro- jected forward into so sharp a mass. In a type like Heterodontus it stands out in a particularly striking way. In many otlier forms, on the contrary, the median olfactory nucleus is poorly developed. The pallial eminences also are very differently developed in different Elas- mobranclis. In Scynuiu.^ again they are sharply defined as prominent dorsal mounds l)uilt up of masses of cells (, fig. 213a). In other forms these lobes are less well developed (Acanthias); while in still others they are scarcely discernible. The diencephalon (di.) in general maj^ be said to be narrow from side to side and show no marked swellings. In some forms it is well seen from above, as in Heterodontus (fig. 210) and Raja (fig. 211b) . In all such occurrences the brain stem is well drawn out in this region. In those Elasmobranchs in which a
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