hird outgrowth (paraphysis) in front of these. On the
floor of the thalamencephalon the blind pouch-like infundibulum is in
contact with the pituitary body, an outgrowth from the combined
pituitary and olfactory pouch, which in the adult opens on to the top
of the head just in front of the pineal area. The anterior closed end
of the nerve-tube, in front of the foramina of Munro or openings from
which the hemispheres have grown out, is known as the _lamina
terminalis_, and in this is seen a little white commissure, connecting
the hemispheres of opposite sides and belonging entirely to the
telencephalon, known as the anterior commissure. The roof of the
telencephalon is mainly epithelial, and contains no traces of cortical
structure. In the posterior part of the roof of the thalamencephalon
is the small posterior commissure (Ahlborn, _Zeits. wiss. Zool._ Bd.
xxxix., 1883, p. 191). In the Elasmobranch Fish, such as the sharks
and rays, the cerebellum (_Cer._ fig. 17) is very large and contains
the layers found in all the higher vertebrates. In the mesencephalon
fibres corresponding with those of the fillet of higher vertebrates
can be seen, and there is a nucleus in the hinder part of the _corpora
bigemina_ foreshadowing the separation into corpora quadrigemina.
There is only one pineal stalk in the roof of the thalamencephalon,
and the ganglia habenulae--very constant structures in the vertebrate
brain--are not so marked as in Petromyzon, but are, as usual,
connected with the olfactory parts of the cerebrum, with the surface
of the optic lobes (_tectum opticum_), and with the _corpus
interpedunculare_ (Meynert's bundle). They are united across the
middle line by a small _superior_ or _habenular commissure_. In the
floor of the thalamencephalon are two masses of ganglionic tissue, the
optic thalami. The infundibulum dilates into two rounded bodies, the
_lobi inferiores_, while the pituitary body or _hypophysis cerebri_
has two lateral diverticula known as _sacci vasculosi_. Ganglia
geniculata are found for the first time in connexion with the optic
tracts in the lower part of the thalamus. The olfactory lobes (fig.
17, _Olf. Bulb_) are very large and often separated by long stalks
from the cerebral hemispheres, which are comparatively much larger
than those of the Cyclostomata; their roof or pallium is nervous, but
devoid of cortical structure, while in th