In the Cyclostome _Petromyzon_ there is a small prosencephalon with an undivided prosocoele, and on each side of it a small cerebral hemisphere which appears as a mere appendage to the much larger olfactory lobe (Fig. 211). The prosocoele divides in front into two outwardly directed branches, and of the two diverticula into which each branch divides one extends as a lateral ventricle into the hemisphere of its side, and the other as a rhinocoele into the corresponding olfactory lobe. The ganglia habenulae are unusually large, the right one being larger than the left. The optic lobes are large, but not obviously double. So small is the cerebellum that it seems to be little more than a narrow transverse band crossing the fore-part of the fourth ventricle. The roof of the brain is largely epithelial, especially in the prosencephalon, the thalamencephalon, and the hind-brain. Over these epithelial areas the pia mater is unusually vascular and forms a series of "choroid plexuses." The ventricular system is complete and continuous. By contrast with the Lamprey the brain of _Myxine_[442] is very primitive, more so perhaps than in any other Craniate (Fig. 212). In a dorsal view the brain is divided into four pairs of laterally expanded and longitudinally compressed lobes by a median longitudinal fissure and three transverse fissures. The two anterior lobes are little more than the thickened anterior wall of the thalamencephalon, although, judging from their histological structure, they represent a very imperfectly differentiated prosencephalon and olfactory lobes. The second and largest pair constitute the thalamencephalon. The last two pairs of lobes represent a transversely divided pair of optic lobes, or "corpora quadrigemina." There is a large medulla oblongata with a pair of corpora restiformia, but the {373}cerebellum is entirely absent. The ventricles are subject to some individual variation. Third and fourth ventricles are generally recognisable, either as isolated cavities or connected by a remnant of the mesocoele. In the feeble development of the prosencephalon, in the striking preponderance of the mid-brain over the rest of the brain, and in the absence of a cerebellum, _Myxine_ is unique amongst Craniates. [Illustration: FIG. 213.—The brain of a Dog-Fish (_Scyllium canicula_). A, dorsal view; B, ventral view. The choroid plexuses covering the roof of the third and fourth ventricles have been removed. _b.o_, Olfactory lobe; _ep_, origin of the stalk of the pineal body; _f.b_ (in A), prosencephalon; _f.b_ (in B), cerebral hemispheres; _fr_, fourth ventricle; _h.b_, cerebellum; _h.p_, pituitary body; _i.f_, lobi inferiores; _m.b_, optic lobes; _m.d_, medulla oblongata; _sc_, saccus vasculosus; _th_, thalamencephalon; _t.o_ (i) olfactory peduncle; i.-x. cranial nerves. (From Wiedersheim.)]