234 EDWARD PHELPS ALLIS, JR. 
In a 20-mm. specimen of Cottus scorpeus I now find the pro- 
otic portion of the myodome separated from the cavum cere- 
brale cranii by membrane only, no cartilaginous or osseous pro- 
otic bridge being as yet developed. In its basioccipital portion 
the myodome lies in a groove on the ventral surface of the basi- 
occipital (fig. 18), which opens posteriorly between the diverg- 
ing hind ends of the parasphenoid, but is there closed ventrally 
by membrane which extends horizontally between those ends. 
This part of the myodome lodges the hind ends of the recti 
externi, the two muscles being separated from each other by a 
delicate vertical membrane. Proceeding anteriorly in the sec- 
tions, the thin cartilage forming the roof of the myodomic 
groove runs gradually into membrane (fig. 17), the entire basis 
cranii thus here being perforated by a longitudinal opening 
that might be considered to be a fenestra ventralis myodomus. 
This is, however, not the case, for the recti externi lie definitely 
in this opening and not above it. The bounding walls of the 
opening accordingly represent the side walls of the myodomic 
groove, and the space between the ventral edges of the side walls 
alone represents the fenestra ventralis myodomus. The space 
between the dorsal edges of the opening is a perforation of the 
floor of the primordial cranium, and the membrane extending 
horizontally between the edges forms part of the floor of the 
cavum cerebrale cranii and also the roof of the dorsal compart- 
ment of this basioccipital portion of the myodome. The recti 
externi lie between this membrane and the parasphenoid, and 
they are still separated from each other by a median vertical 
membrane. The saccus vasculosus is large, lies in the cavum 
cerebrale cranii, and projects posteriorly slightly beyond this 
point. 
Proceeding anteriorly in the sections to the region between 
the saccus vasculosus and the hypophysis (fig. 16), the mem- 
branous roof of the myodome becomes somewhat arched, and 
it now has its attachment, on either side, on the dorso-internal , 
surface of the cartilage of the basis cranii, at some distance 
dorsolateral to the midventral perforation of the cartilage, that 
perforation now being definitely a fenestra ventralis myodomus. 
