The brain of _Chondropterygians_ (Fig. 43) is more developed than that of all other fishes, and distinguished by well-marked characters. These are, first, the prolongation of the olfactory lobes into more or less long pedicles, which dilate into great ganglionic masses, where they come into contact with the olfactory sacs; secondly, the space which generally intervenes between prosencephalon and mesencephalon, as in some Ganoids; thirdly, the large development of the metencephalon. The hemispheres are generally large, coalescent, but with a median, longitudinal, dividing groove. Frequently their surface shows traces of gyrations, and when they are provided with lateral ventricles, tubercles representing _corpora striata_ may be observed. The olfactory pedicles take their origin from the side of the hemispheres, and are frequently hollow, and if so, their cavity communicates with the ventricle of the hemisphere. The optic lobes are generally smaller than the hemispheres, coalescent, and provided with an upper median groove like the prosencephalon. At their base a pair of lobi inferiores are constant, with the hypophysis and _sacsus vasculosus_ (a conglomeration of vascular loops without medullary substance) between them. The cerebellum is very large, overlying a portion of the optic lobes and of the sinus rhomboidalis, and is frequently transversely grooved. The side-walls of the fourth ventricle, which are formed by the corpora restiformia, are singularly folded, and appear as two pads, one on each side of the cerebellum (_lobi posteriores_ s. _lobi nervi trigemini_). [Illustration: Fig. 44.--Brain of Bdellostoma. (Enlarged, after Müller.) I., Upper; II., Lower aspect. Letters as in Fig. 45.] [Illustration: Fig. 45.--Brain of Petromyzon. (Enlarged, after Müller.) I., Upper; II., Lower aspect. _a_, Medulla oblongata; _ac_, nerv. acusticus; _b_, corpus restiforme or rudimentary cerebellum; _d_, lobus ventriculi tertii; _d’_, entrance into the third ventricle; _c_, hypophysis; _fa_, nerv. facialis; _g_, nerv. opticus; _h_, hemisphere; _hy_, nerv. hypoglossus (so named by Müller); _i_, lobus olfactorius; _k_, sinus rhomboidalis; _l_, epiphysis; _m_, nerv. oculo-motorius; _q_, corpora quadrigemina; _tr_, nerv. trigeminus; _tro_, nerv. trochlearis; _v_, nerv. vagus.]