MYODOME AND TRIGEMINO-FACIALIS CHAMBER 281 
transversus is found in these fishes, as in the Selachii, the con- 
ditions are probably strictly similar. 
DIPNOI 
In Ceratodus, the so-called pars ascendens of the anterior 
process of the palatoquadrate of Greil’s (13) descriptions forms 
the lateral wall of a space which, in an earlier work (Allis, ’14 ¢), 
I showed to be the homologue of the trigemino-facialis chamber 
of the Holostei. In early embryos of Ceratodus this chamber 
has anterior and posterior openings which Greil calls, respec- 
tively, the foramen sphenoticum commune and the foramen 
praeoticum basicraniale. In older embryos the foramen sphen- 
oticum commune becomes separated into four parts by bars of 
cartilage developed in the connective tissues surrounding the 
nerves and vessels which traverse the foramen. One of these 
parts, called by Greil the foramen sphenoticum majus, transmits 
all the branches of the nervi maxillo-mandibularis and lateralis 
trigemini and the vena and arteria temporalis, the latter artery 
being the carotis externa of my descriptions of other fishes. <A 
second foramen, called the foramen sphenoticum minus, trans- 
mits the nervus profundus and the vena capitis media, this 
latter vein being also called the vena pterygoidea. A third 
foramen, called the foramen hypoticum, transmits the nervus 
oticus trigemini; the fourth foramen transmitting the nervus 
abducens. The posterior opening of the chamber, the fora- 
men praeoticum basicraniale, does not undergo subdivision in 
the oldest embryos considered by Greil, and it is traversed by the 
nervus facialis, the ramus palatinus facialis, the arteria tem- 
poralis (carotis externa), and the vena capitis lateralis; the latter 
vein being a posterior continuation of the vena capitis media 
(pterygoidea), and the two together forming the vena jugularis of 
my descriptions of other fishes. The floor of the trigemino- 
facialis chamber is formed by the processus basalis of the pala- 
toquadrate, and the palatinus facialis, after issuing through 
the posterior opening of the chamber, runs forward ventral to 
this floor, between it and the underlying parasphenoid. 
