260 EDWARD PHELPS ALLIS, JR. 
municate with the middle section through the fenestra basi- 
cranialis posterior, which lies between the anterior ends of the 
parachordals and apparently corresponds to the posterior por- 
tion of the intertrabecular fissure of Scheip’s descriptions. The 
recti externi pass through this fenestra, and, beyond it, lie be- 
neath the basis cranii. There, as they increase in size, they 
are said to push both the middle portion of the basis cranii 
upward and the parasphenoid downward. The basal plate, 
formed by the parachordals and the enclosed notochord, then 
thickens along each lateral surface of these muscles, and so forms 
the lateral walls of this section of the myodome. 
Comparing the conditions in this fish with those in Gaster- 
osteus, it is seen that, in both fishes, the anterior portions of 
the parachordals lie, when first formed, at a certain distance 
lateral to the anterior end of the notochord, which projects anteri- 
orly between them. In later stages of both fishes these project- 
ing portions of the parachordals are said, by both Gaupp and 
Swinnerton, to be depressed, but the figures given by both show 
that this depression effects only the mesial edges of the para- 
chordals, their lateral portions retaining their primitive posi- 
tions in the level of the notochord. Between these higher lying 
portions of the cartilages, the prootic bridge is later developed. 
How this bridge is developed in Gasterosteus is not stated by 
Swinnerton. In Salmo Gaupp says it is formed by the chon- 
drification of a part of the membrane forming the roof of the 
middle section of the myodome and also the floor of the cavum 
cerebrale cranii. Gaupp says it lies, when first formed, an- 
terior to and above the tip of the notochord, and it is shown, 
in one of his figures, separated from that tip by the anterior 
portion of an open space that is prolonged posteriorly on either 
side of the anterior end of the notochord. The posterior portion 
of this space may possibly form part of the fenestra basicranialis 
posterior of Gaupp, but its anterior portion certainly does not, for 
Gaupp says that this fenestra les between the edges of the bent- 
down parachordals and gives passage to the recti externi from 
the middle to the posterior section of the myodome. There 
are, then, four distinctly different fenestrae in this myodomic 
