346 J. B. JOHNSTON 
In order to demonstrate the pillars of the velar arch in this 
form the writer has made a plate reconstruction of the dorsal 
part of the telencephalon and diencephalon. Owing to the com- 
pression of the dorsal sac by the epiphysis and parapineal body the 
velum shows to best advantage in sagittal sections and the recon- 
struction was made from these (fig. 8). The model was made 
from the left side of the brain and included the recessus neuropori- 
cus in front and a part of the nucleus habenulae behind. In 
order to see as much of the velum as possible the model has been 
drawn from a medio-ventro-caudal direction. This figure should 
be compared with figure 3, which shows the left half of the fore- 
brain from a model of another specimen of Petromyzon dorsatus 
(ammocoetes). The cut surface along the upper border of the 
figure corresponds very nearly to the median sagittal plane. In 
this plane is seen the fold described by Sterzi as the velum trans- 
versum. Extending caudo-laterally from this above the primor- 
dium hippocampi is a fold of the tela which continues without 
interruption into the extreme caudo-lateral angle of the dorsal 
portion of the ventricle. The deep cleft in which the velum is 
seen in the figure is the recessus praehabenularis described above. 
In the depth of this recess the lateral pillar of the velar arch is 
attached to asmall ridge which is identified as the eminentia 
thalami by the presence in it of the stria medullaris. As the 
model is viewed from an unusual angle, the relations will perhaps 
be more clear by comparison with figures 23 to 27, which represent 
five sections of the series from which the model was constructed. 
The relations of the recessus praehabenularis and of the eminentia 
thalami in Lampetra are shown in figures 14 to 22. Here the 
sulcus praehabenularis is narrower than in the ammocoetes 
because of the enlargement and crowding of the surrounding parts. 
The disposition of the velum transversum in Petromyzon dorsatus 
is essentially the same as that in ganoids and teleosts. The 
velum is not only inclined far forward as in the latter fishes, but 
owing to the depression of the dorsal sac by the overlying epiphy- 
sis, its middle part is pressed down far below the plane of attach- 
ment of its pillars. The whole course of the velum is diagram- 
matically represented in figure 28 as it might be seen in the dorsal 
