226 EDWARD PHELPS ALLIS, JR. 
externus. The ramus palatinus facialis also perforates the roof 
and traverses the dorsal compartment in order to reach the 
ventral one, but it is separated from the central cavity of the 
dorsal compartment by a membrane, apparently of skeletog- 
enous character, the nerve thus probably lying morphologically 
in the wall of this compartment of the myodome and not actu- 
ally traversing it. 
The recti externi, after issuing through the posterior opening 
of the dorsal compartment of the myodome, extend posteriorly a 
certain distance, there lying in a part of the aortal groove which 
differs slightly in character from the part posterior to it. This 
anterior part of the groove is, however, so evidently an anterior 
prolongation of its posterior portion that the two parts must 
be of similar origin, and as the posterior portion of the groove 
has certainly not been developed in any relation whatever to 
any of the muscles of the eyeball, it is certain that the anterior 
portion also has not been so developed. This is, furthermore, 
confirmed by the conditions in Polypterus, in which there is no 
functional myodome, but there is both a cavity corresponding 
to the dorsal compartment of the myodome of Hyodon and a 
closed and wholly separate canal lodging the cranial portion 
of the aorta and corresponding to the aortal groove of Hyodon. 
This myodomic cavity and aortal canal have both been referred 
to and discussed in certain of my earlier works (Allis,’08 a,’09), 
and I now find, on reexamining my sections of a small specimen 
of this fish, that the enclosing walls of the aortal canal give even 
more positive evidence of having been formed by vertebral proc- 
esses than do the walls of the groove of Hyodon. 
There thus seems little doubt that the bounding walls of the 
aortal groove of Hyodon are formed by processes similar to 
those which enclose the haemal canal of the tail, and that those 
bounding walls are formed either by the entire ventrolateral 
processes of vertebrae which here have been incorporated in the 
neurocranium, or by aortal supports developed in relation to 
those processes; and if the walls of this groove are so formed, it 
would seem as if the side walls of the prootic portion of the dor- 
sal compartment of the myodome, evidently an anterior con- 
