MYODOME AND TRIGEMINO-FACIALIS CHAMBER 249 
relations as in the stickleback. It is probable, therefore, that in this 
fish also a process of depression and secondary growth goes on on 
either side of the fossa and below the prootic; but that, whereas in the 
other types the fossa persists and transmits the eye muscles back again 
out of the cranial cavity beneath the basis cranii, in Amia it disappears, 
owing to continuous cartilaginous growth. As far back as the so-called 
prootic bridge these muscles may be said to run in an actual derivative 
of the cranial cavity; behind that they run in an extracranial space 
secondarily enclosed. 
The myodome of Gasterosteus is thus conceived by Swinner- 
ton to be a space, the anterior portion of which is bounded 
laterally by the bent-down anterior prolongations of the para- 
chordals, and the posterior portion by secondary ventral down- 
growths of the parachordals posterior to those anterior pro- 
longations. These two portions of the myodome are thus of 
totally different origin, and the anterior portion is considered, 
because of its relations to the parachordals, to be an actual 
derivative of the cavum cerebrale cranii. The figures given 
show that it lodges the pituitary body, and that it is prechordal 
in position, but, as the arteries and veins of the region are not 
shown or particularly described, it is impossible to compare 
the conditions here with those in the fishes that I have con- 
sidered above. I have accordingly examined this region in a 
series of transverse sections of a 40-mm. specimen of this fish, 
and as the conditions there present certain new features, they 
will be quite fully described. 
In this 40-mm. specimen of Gasterosteus I find the recti 
superiores, inferiores, and interni all arising from a thick median 
vertical membrane which descends from the ventral surface of 
the anterior portion of an unusually large membranous pitu- 
itary sac, the recti interni having their origins posteroventral 
to the recti superiores and inferiores. Posterior to the points 
of origin of these muscles, the large hypophysis projects ven- 
trally into the membranous pituitary sac, which lies dorsal to, 
and in large part posterior to, the dorsoanterior edge of that 
transverse ridge on the dorsal surface of the parasphenoid which 
Swinnerton calls its median process. This process begins near 
the hind edges of the ascending processes of the parasphenoid, 
