Selachians.--No parapineal organ is present. The pineal body (except in _Torpedo_ where it is absent) is in the form of a long slender tube ending in front in a dilated bulb lying near the front end of the brain in close contact with, or enclosed in, a definite foramen in the cranial roof. Holocephali and Crossopterygii.--Here also the pineal body is long and tubular: at its origin it passes dorsalwards or slightly backwards behind the large dorsal sac. Actinopterygian Ganoids resemble Selachians on the whole. In _Amia_ a parapineal organ is present, and it is said to lie towards the left side and to be connected by a thick nerve with the _left_ habenular ganglion (cf. _Petromyzon_, article CYCLOSTOMATA). This is adduced to support the view that the pineal and parapineal bodies represent originally paired structures. Teleostei.--A parapineal rudiment appears in the embryo of some forms, but in the adult only the pineal organ is known to exist. This is usually short and club-shaped, its terminal part with much folded wall and glandular in character. In a few cases a parietal foramen occurs (_Callichthys_, _Loricaria_, &c.). Dipneusti.--The pineal organ is short and simple. No parapineal organ is developed. The dorsal sac is formed by that part of the roof of the thalamencephalon lying between the habenular commissure and the region of the velum. In some cases a longitudinal groove is present in which the pineal organ lies (Dipneusti). In the Crossopterygians the dorsal sac is particularly large and was formerly mistaken for the pineal organ. The _velum transversum_ is a transverse, inwardly-projecting fold of the roof of the primitive fore-brain in front of the dorsal sac. To those morphologists who regard the hemisphere region or telencephalon as a primitively unpaired structure the velum is an important landmark indicating the posterior limit of the telencephalon. Those who hold the view taken in this article that the hemispheres are to be regarded as paired outpushings of the side wall of the primitive fore-brain attribute less morphological importance to the velum. Physiologically the velum is frequently important from the plexus of blood-vessels which passes with it into the III. ventricle. In _Petromyzon_ and _Chimaera_ the velum is not developed. In Dipnoans there are present in its place _paired_ transverse folds which are probably merely extensions backwards of the lateral plexuses.