MYODOME AND TRIGEMINO-FACIALIS CHAMBER 253 
anterior to this point, formed the roof of the myodome, is now 
replaced by membrane bone which forms the anterior portion 
of the prootic bridge, and the nervus abducens perforates it, 
on either side, to enter the dorsal compartment of the myo- 
dome. Farther posterior in the sections, a median plate of 
cartilage appears in the prootic bridge, enclosed between dor- 
sal and ventral plates of perichondrial bone, and in this trans- 
verse plane the cartilage in the ventral ends of the ventral proc- 
esses of the prootics disappears and is replaced by membrane 
only. The ventral processes of the prootics are accordingly 
now formed by short processes of bone, partly of membrane 
and partly of perichondrial origin, that are prolonged ventrally 
by membranes, continuous ventrally with the lateral edges 
of the parasphenoid. Proceeding posteriorly, the median plate 
of cartilage expands laterally, on either side, and becomes the 
cartilaginous basis cranii, here still enclosed, on either side, 
between plates of perichondrial bone which form parts of the 
prootics. The myodome still continues onward, in a median 
groove on the ventral surface of this cartilage, there lodging 
the recti externi and being bounded laterally in part by mem- 
brane only and ventrally by the parasphenoid. 
At the extreme hind end of the myodome a circle of bone appears 
in the sections, this bone forming part of the basioccipital and 
lying in the groove on the ventral surface of the cartilaginous basis 
crani. From this shell of bone a median plate is sent down- 
ward between the diverging hind ends of the parasphenoid, and 
the shell of bone then fuses with perichondrial bone developed 
in relation to the overlying cartilage and forming part of the 
basioccipital. The conditions are thus here practically as de- 
seribed and figured by Swinnerton in his 16-mm. specimen of 
this fish (1. c., fig. 36, pl. 30). 
The myodome of Gasterosteus is thus strictly comparable to 
that in the other Teleostei described, except that it has a greater 
anterior extension than it has in any of them, Cottus and Clin- 
ocottus excepted, this being due to the ossification, as part of 
each ascending process of the parasphenoid, of tissues representing 
the pedicel of the alisphenoid. The parasphenoid has under- 
JOURNAL OF MORPHOLOGY, VOL. 32, NO. 2 
