248 
The pituitary fossa and the trigemino-facialis chamber of Amia, 
fused to form the so-called myodome of the fish, thus each consists 
of two portions, one of which is the homologue of the corresponding, 
intramural space of teleosts, Lepidosteus and selachians, while the 
other is a space that is extracranial in the latter fishes; and this must 
also be true of these two chambers, where found, not only in all fishes 
in which the alisphenoid bone may have a parasphenoid leg, but 
also in all higher vertebrates in which the homologue of that leg is 
found, that homologue apparently being either the antipterygoid or 
the epipterygoid of current descriptions. 
The several steps in the development of the trigemino-facialis 
chamber of fishes, as above described, are shown in the accompanying 
purely diagrammatic figures. In these figures the trigeminus and 
facialis nerves are not shown, the sectional views given in the figures 
being assumed to pass between those two nerves. From these figures, 
together with the descriptions given, it is seen that the trigemino- 
facialis chamber of ganoids and teleosts is the result of the more or 
less complete fusion of the canal for the internal jugular vein of 
selachians with the external carotid and facialis canals and the trı- 
gemino-pituitary fossa of those same fishes. And in those higher 
vertebrates in which a corresponding chamber is found it seems quite 
certainly to be the homologue of the internal jugular vein of fishes, 
rather than the trigeminus and facialis nerves, that has primarily 
determined the extent and character of the chamber. Of these higher 
vertebrates it will be sufficient, for the purposes of this paper, to 
refer briefly to amphibians, reptiles and Echidna. 
In amphibians and in Lacerta, the internal jugular vein of fishes 
is represented by a vein that is formed by a combination of certain 
sections of the vena cardinalis anterior with certain sections of the 
vena capitis lateralis, and both the vein so formed and the external 
carotid artery lie external to the side wall of the chondrocranium. 
There is accordingly no enclosed trigemino-facialis chamber in the 
chondrocrania of these animals. The lateral wall of the anterior 
portion of this chamber, as found in Amia, is, however, represented 
in the ascending process of the palato-quadrate of amphibians and 
in the columella of Lacerta (Anus, 1914), and it is possible that there 
are, posterior to these structures, eonnective or membranous tissues 
that enclose the vein and artery, one or both, and that represent the 
lateral wall of the posterior portion of the chamber. In certain rep- 
