In Elasmobranchs among Fishes the brain attains a much higher grade of structure. In _Scyllium_ (Fig. 213) there is a large prosencephalon, and directly in front of it a pair of imperfectly differentiated cerebral hemispheres, while from its antero-lateral regions the large olfactory lobes arise. The prosocoele divides in front into four diverticula, of which the two {374}inner ones extend into the hemispheres as lateral ventricles, and the two outer as rhinocoeles into the olfactory lobes (Fig. 214). In connexion with the infundibulum there is a pair of sacci vasculosi, consisting mainly of gland-tubules, opening into the infundibular cavity.[443] The cerebellum is exceptionally large, but it does not form a "valvula cerebelli." Large ear-like corpora restiformia are present. The third and fourth ventricles alone retain an epithelial roof in relation with choroid plexuses. In all essentials the brain of the Holocephali is a repetition of the Elasmobranch type, more especially of the elongated form seen in _Notidanus_. Indications of a higher grade of structure are, however, to be seen in the reduction of the prosencephalon which, with its prosocoele, is now scarcely distinguishable from the thalamencephalon and its ventricle; and in the more complete differentiation of the cerebral hemispheres from one another and from the rest of the brain. Large frilled corpora restiformia are conspicuous structures on each side of the medulla oblongata. Besides the usual intra-cranial pituitary body, there is also a separate extra-cranial portion lodged in a pit on the ventral surface of the basis cranii: in the embryo the two are continuous. [Illustration: FIG. 214.—Horizontal longitudinal section of brain of _Chiloscyllium_, to show the ventricles; semi-diagrammatic. _cer_, Origin of cerebellar ventricle or epicoele; _dia_, third ventricle; _iter_, mesocoele; _meta_, fourth ventricle; _opt_, optocoele, or cavity of an optic lobe; _para_, lateral ventricles; _pros_, prosocoele; _rh_, rhinocoele. (From Parker and Haswell.)] In the Teleostomi the brain is distinctly of a more primitive type than in any other Fishes (Fig. 215).[444] The most striking feature is the absence of cerebral hemispheres, the evolution of the primary fore-brain proceeding no farther than the formation of an undivided prosencephalon with a non-nervous roof, and a prosocoele which forms a continuous cavity with the third ventricle, or at the most is only separated from it by an infolding of the epithelial roof or velum transversum.