The brain of _Ganoid fishes_ shows great similarity to that of the Teleostei; however, there is considerable diversity of the arrangement of its various portions in the different types. In the Sturgeons and _Polypterus_ (Fig. 42) the hemispheres are more or less remote from the mesencephalon, so that in an upper view the crura cerebri, with the intermediate entrance into the third ventricle (_fissura cerebri magna_), may be seen. A vascular membranous sac, containing lymphatic fluid (_epiphysis_), takes its origin from the third ventricle, its base being expanded over the anterior interspace of the optic lobes, and the apex being fixed to the cartilaginous roof of the cranium. This structure is not peculiar to the Ganoids, but found in various stages of development in Teleosteans, marking, when present, the boundary between prosencephalon and mesencephalon. The lobi optici are essentially as in Teleosteans. The cerebellum penetrates into the ventriculus lobi optici, and extends thence into the open sinus rhomboidalis. At its upper surface it is crossed by a commissure formed by the _corpora restiformia_ of the medulla. [Illustration: Fig. 42.--Brain of Polypterus. (After Müller.) I., Upper; II., Lateral; III., Lower aspect. _a_, Medulla; _b_, corpora restiformia; _c_, cerebellum; _d_, lobi optici; _e_, hypophysis; _f_, fissura cerebri magna; _g_, nervus opticus; _g_’, chiasma; _h_, hemispheres; _i_, lobus olfactorius; _k_, sinus rhomboidalis (fourth ventricle).] As regards external configuration, the brain of _Lepidosteus_ and _Amia_ approach still more the Teleosteous type. The prosencephalon, mesencephalon, and metencephalon are contiguous, and the cerebellum lacks the prominent transverse commissure at its upper surface. The sinus rhomboidalis is open. The brain of the _Dipnoi_ shows characters reminding us of that of the Ganoids as well as the Chondropterygians, _Ceratodus_ agreeing with _Protopterus_ in this respect, as in most other points of its organisation. The hemispheres form the largest part of the brain; they are coalescent, as in Sharks, but possess two lateral ventricles, the separation being externally indicated by a shallow median groove on the upper surface. The olfactory lobes take their origin from the upper anterior end of the hemispheres. Epiphysis and hypophysis well developed. The lobi optici are very small, and remote from the prosencephalon, their division into the lateral halves being indicated by a median groove only. The cerebellum is very small, overlying the front part of the sinus rhomboidalis. [Illustration: Fig. 43.--Brain of Carcharias. (After Owen.) _ac_, Nerv. acusticus; _b_, corpus restiforme; _c_, cerebellum; _d_, lobus opticus; _e_, hypophysis; _g_, nervus opticus; _h_, hemisphere; _i_, lobus olfactorius; _i’_, olfactory pedicle; _k_, nerv. olfactorius; _l_, epiphysis; _m_, nerv. oculo-motorius; _tr_, nerv. trigeminus; _v_, nerv. vagus.]