MYODOME AND TRIGEMINO-FACIALIS CHAMBER 209 
always limited to the caudal region, as the conditions in Aci- 
penser show (Bridge, ’(04, fig. 115, p. 200). 
Accordingly, with this idea definitely in view, I have care- 
fully examined the myodome, not only in this series of sections 
of Hyodon and in a single prepared skull that I have of the adult 
fish, but also in all other embryos and adults of the Holostei 
and Teleostei that I at present have at my disposal that seem 
to be of interest in this connection, and the result has been not 
only to strongly favor this interpretation of the conditions, 
but also to give a conception of the myodome itself and the bones 
related to it quite different from that I formerly held. The 
functional myodome, as found in the fishes examined, will first 
be quite fully described, and then comparison made with the 
descriptions of the corresponding parts in certain other fishes 
and in certain of the higher vertebrates. 
The preliminary examination of the serial sections used in 
connection with the work was wholly done by my assistant, 
Mr. John Henry, camera drawings being made of many sec- 
tions of each series. The drawings used for the figures are by 
my assistant, Mr. Jujira Nomura. 
HYODON TERGISUS 
The myodome of Hyodon has never been described, so far as 
I can find, except by Ridewood, (’04), who only says that, in 
Hyodon alosoides: ‘‘The parasphenoid underlies but a small 
portion of the basioccipital, and the eye-muscle canal opens 
at its posterior end by an oval foramen.”’ 
In my skull of the adult Hyodon tergisus, the anterior open- 
ing of the myodome is triangular and unusually large and tall 
for the size of the skull. Posterior to this opening the myodome 
diminishes rapidly in size, and finally becomes continuous with 
an open groove on the ventral surface of the basioccipital. This 
groove extends to the hind end of the basioccipital and there cuts 
through the ventral edge of the vertebra-like hind end of the bone 
to open upon its posterior surface. This open groove forms no 
part of the myodome as described by Ridewood, his myodome 
ending at the oval foramen described by him, which leads from 
