In the Teleostomes the mass of nervous matter which in other groups forms the hemispheres does not undergo any pushing outwards except as regards the small olfactory lobes. On the contrary, it remains as a great thickening of the lateral wall of the thalamencephalon (the so-called basal ganglia), additional space for which, however, may be obtained by a considerable increase in length of the fore-brain region (cf. fig. 30, A) or by actual involution into the third ventricle (_Polypterus_).[46] The great nervous thickenings of the thalamencephalic wall bulge into its cavity and are covered over by the thin epithelial roof of the thalamencephalon which is as a consequence liable to be confused with the pallium or roof of the hemispheres with which it has nothing to do: the homologue of the pallium as of other parts of the hemisphere is contained within the lateral thickening of the thelamencephalic wall, not in its membranous roof.[47] Associated with the parts of the fore-brain devoted to the sense of smell (especially the corpora striata) is the important system of bridging fibres forming the anterior commissure which lies near the anterior end of the floor, or in the front wall, of the primitive fore-brain. It is of great interest to note the appearance in the _Dipnoans_ (_Lepidosiren_ and _Protopterus_) of a corpus callosum (cf. fig. 30 B) lying dorsal to the anterior commissure and composed of fibres connected with the pallial region of the two hemispheres. _Sense Organs._--The olfactory organs are of special interest in the Selachians, where each remains through life as a widely-open, saccular involution of the ectoderm which may be prolonged backwards to the margin of the buccal cavity by an open oronasal groove, thus retaining a condition familiar in the embryo of the higher vertebrates. In Dipnoans the olfactory organ communicates with the roof of the buccal cavity by definite posterior nares as in the higher forms--the communicating passage being doubtless the morphological equivalent of the oronasal groove, although there is no direct embryological evidence for this. In the Teleostomes the olfactory organ varies from a condition of great complexity in the Crossopterygians down to a condition of almost complete atrophy in certain Teleosts (Plectognathi).[48]