344 J. B. JOHNSTON 
is the point of attachment of the velum to the massive walls which 
determines the boundary between telencephalon and diencephalon. 
The velum transversum is a fold of the tela chorioidea having the 
form of an arch whose pillars rest on the massive lateral walls. 
This point of attachment is the meeting place of the taenia thalami 
and taenia fornicis. The position of the velar arch in the median 
plane depends upon the form of the tela chorioidea of the third 
ventricle as affected by the general form of the brain and the 
pressure or traction exerted upon the tela by surrounding struc- 
tures. Thus in the selachian brain the velum is nearly transverse 
and vertical in position (fig. 1) and is attached to the lateral walls 
just in front of the habenular bodies. In certain ganoids, on the 
other hand, the velum is a very deep fold which is inclined for- 
ward at an angle greater than 45 degrees. The pillars of the 
velum are attached to the lateral walls just rostral to the habenu- 
lar bodies exactly as in selachians (fig. 2). The point of attach- 
ment of the velum to the lateral walls may be more sharply defined 
with reference to the internal structure of the massive wall. It 
is just at the point of attachment of the velum that the several 
categories of fibers which make up the stria medullaris converge 
into a compact bundle to ascend to the habenular nucleus and 
commissure (commissura superior Osborn). In amphibians and 
reptiles this portion of the lateral wall, to which the velum is 
attached and which is traversed by the stria medullaris, Herrick 
(10, p. 419) has ealled the eminentia thalami. This low eminence 
is clearly seen in figures 1 and 2, but is not lettered. 
A careful study of the velum transversum in Lampetra shows 
that it is attached as in fishes and amphibians immediately rostral 
to the habenular bodies and that its point of attachment is the 
dorsal border of the eminentia thalami. 
As is well known, the right habenular body is very much larger 
than the left, and each is bounded ventrally by a deep groove, the 
sub-habenular sulcus (figs 8, 5, 6). At its rostral end this deep 
sulcus leads into the dorsal sac. Here it meets with a vertical 
suleus which descends in the brain wall and curves forward to 
enter the interventricular foramen. (figs.5,6,suleus limitans hippo- 
campi). The common space formed by the union of these two 
