The brain of the _Cyclostomes_ (Figs. 44, 45) represents a type different from that of other fishes, showing at its upper surface three pairs of protuberances in front of the cerebellum; they are all solid. Their homologies are not yet satisfactorily determined, parts of the Myxinoid brain having received by the same observers determinations very different from those given to the corresponding parts of the brain of the Lampreys. The foremost pair are the large olfactory tubercles, which are exceedingly large in _Petromyzon_. They are followed by the hemispheres, with a single body wedged in between their posterior half; in _Petromyzon_, at least, the vascular tissue leading to an epiphysis seems to be connected with this body. Then follows the lobus ventriculi tertii, distinctly paired in Myxinoids, less so in _Petromyzon_. The last pair are the _corpora quadrigemina_. According to this interpretation the cerebellum would be absent in Myxinoids, and represented in _Petromyzon_ by a narrow commissure only (Fig. 45, _b_), stretching over the foremost part of the sinus rhomboidalis. In the Myxinoids the _medulla oblongata_ ends in two divergent swellings, free and obtuse at their extremity, from which most of the cerebral nerves take their origin. The _Nerves_ which supply the organs of the head are either merely continuations or diverticula of the brain-substance, or proper nerves taking their origin from the brain, or receiving their constituent parts from the foremost part of the spinal chord. The number of these spino-cerebral nerves is always less than in the higher vertebrates, and their arrangement varies considerably. A. _Nerves which are diverticula of the brain_ (Figs. 41–45). The _olfactory_ nerves (_first pair_) always retain their intimate relation to the hemispheres, the ventricles of which are not rarely continued into the tubercle or even pedicle of the nerves. The different position of the olfactory tubercle has been already described as characteristic of some of the orders of fishes. In those fishes in which the tubercle is remote from the brain, the nerve which has entered the tubercle as a single stem leaves it split up into several or numerous branches, which are distributed in the nasal organ. In the other fishes it breaks up into branchlets spread into a fan-like expansion at the point, where it enters the nasal cavity. The nerve always passes out of the skull through the ethmoid.