{375}[Illustration: FIG. 215.—A, dorsal view of the brain of a Trout (_Salmo fario_); B, a vertical longitudinal section. _c.il_, Commissura interlobularis; _g.h_, ganglion habenulae; _h.c_, habenular commissure; _i.c_, inferior commissure; _l.i_, lobus inferior; _myc_, myelocoele; _p.c_, posterior commissure; _v.o_, valvula cerebelli; _v.t_, velum transversum; _ii_., optic nerve; _v.iii_., _v.iv_., third and fourth ventricles; _v_, _vii_, _viii_, _ix_, _x_, fifth, seventh, eighth, ninth, and tenth cranial nerves; remaining reference letters as in Fig. 210. (A, From Wiedersheim; B, after Haller.)] Amongst other diagnostic characters may be mentioned the predominance of the mid-brain over the other divisions, the anterior extension of the large cerebellum into the mesocoele as a "valvula cerebelli," {376}and the absence of corpora restiformia. This type of brain is most strongly marked in the Teleostei, but in other Teleostomes some, like _Acipenser_,[445] are typically Teleostean in this respect (Fig. 216), while others, such as _Lepidosteus_, have small cerebral hemispheres with lateral ventricles as well as a prosencephalon. [Illustration: FIG. 216.—Vertical longitudinal section of the brain of a Sturgeon (_Acipenser ruthenus_.) _c.p_, Posterior commissure; _c.r_, cranial roof; _mc_, mesocoele; _op.ch_, optic chiasma; _p.ch.p_, posterior choroid plexus; _v.c_, valvula cerebelli; _v.t_, velum transversum; _v.iii_, _v.iv_., the third and fourth ventricles; other lettering as in Fig. 210. (From Goronowitsch.)] The most obvious feature in the brain of the Dipnoi is the great development of the cerebral hemispheres. In this respect these Fishes approach the Amphibia, but in other features of brain-structure they present points of agreement with most other groups of Fishes without being closely related to any one of them. In _Protopterus_[446] (Fig. 217) the hemispheres are quite distinct except behind, and the walls of their spacious lateral ventricles are entirely nervous. Olfactory lobes are sessile on their anterior extremities, and behind and below they enlarge into ventral lobes which probably represent the hippocampal lobes of the higher Vertebrates. A vesicular pineal body at the end of a slender stalk overlies a singular conical projection from the roof of the thalamencephalon or "pineal pillow." {377}[Illustration: FIG. 217.—Dorsal (A), ventral (B), and lateral (C) views of the brain of _Protopterus annectens_. _C_, Cerebellum; _C.H_, cerebral hemisphere; _D.S.E_, branches of the sinus endolymphaticus; _In_, infundibulum; _L.I_,, lobi inferiores; _M.O_, medulla oblongata; _O.L_, olfactory lobe; _Op.L_, optic lobe; _P_, pituitary body; _P.B_, "pineal pillow"; _S.E_, sinus endolymphaticus; _Sp.c_, spinal cord; _Sp.n_, spinal nerve; _Vel_, velum transversum; _Z_, pineal body; IV.V., fourth ventricle; ii., iii., iv., v., vi., vii., viii.1, viii.2, viii.3.4, ix., and x., roots of the cranial nerves. (From Burckhardt.)]