In Teleostei the cerebrum appears to be completely divided into two hemispheres, which are, however, all but solid, the lateral ventricles being only prolonged into their bases. In Dipnoi again there is either (Protopterus, Wiedersheim[517]) a completely separated pair of oval hemispheres, not unlike those of the lower Amphibia, or the oval hemispheres are not completely separated from each other (Ceratodus, Huxley[518], Lepidosiren, Hyrtl[519]); in either case the hemispheres are traversed for the whole length by lateral ventricles which are either completely or nearly completely separated from each other.
In Elasmobranchii the cerebrum is an unpaired though bilobed body, but traversed by two completely separated lateral ventricles, and without a trace of the peculiar membranous roof found in Ganoids.
Not less interesting than the distinguishing characters of the Ganoid brain are those cerebral characters which indicate affinities between Lepidosteus and other groups. The most striking of these are, as might have been anticipated, in the direction of the Teleostei.
Although the foremost division of the brain is very dissimilar in the two groups, yet the hind-brain in many Ganoids and the mid-brain also in Lepidosteus approaches closely to the Teleostean type. The most essential feature of the cerebellum in Teleostei is its prolongation forwards into the ventricles of the optic vesicles as the valvula cerebelli. We have already seen that there is a homologous part of the cerebellum in Lepidosteus; Stannius also describes this part in the Sturgeon, but no such part is represented in Müller's figure of the brain of Polypterus, or described by him in the text.
The cerebellum is in most Ganoids relatively smaller, and this is even the case with Amia; but the cerebellum of Lepidosteus is hardly less bulky than that of most Teleostei.
The presence of tori semicirculares on the floor of the mid-brain of Lepidosteus again undoubtedly indicates its affinities with the Teleostei, and such processes are stated by Stannius to be absent in the Sturgeon, and have not, so far as we are aware, been described in other Ganoids. Lastly we may point to the presence of well-developed lobi inferiores in the brain of Lepidosteus as an undoubted Teleostean character.
On the whole, the brain of Lepidosteus, though preserving its true Ganoid characters, approaches more closely to the brain of the Teleostei than that of any other Ganoid, including even Amia.
It is not easy to point elsewhere to such marked resemblances of the Ganoid brain, as to the brain of the Teleostei.
The division of the cerebrum into anterior and posterior lobes, which is found in Lepidosteus, probably reappears again, as already indicated, in the higher Amphibia. The presence of the peculiar vesicle attached to the roof of the thalamencephalon has its parallel in the brain of Protopterus, and as pointing in the same direction a general similarity in the appearance of the brain of Polypterus to that of the Dipnoi may be mentioned.
There appears to us to be in no points a close resemblance between the brain of Ganoids and that of Elasmobranchii.