THE MYOLOGY OF POLYODON 125 
be on the anterior side except in a very few cases, as for example 
in Amia, where the muscle, already at the point of disappearing 
may be slightly altered in its relations. The several descriptions 
show, however, that the origin and insertions are often very 
nearly or quite on the inner margins of the cartilages concerned. 
If, therefore, the muscles of Polyodon were already differentiated 
and in this condition before the cartilages became flattened, they 
might, it would seem, migrate indifferently to either the posterior 
or anterior sides of the modified branchial cartilages, and the 
condition observed in Polyodon established. If this really repre- 
sents the phylogenetic development of the muscle, then there 
seems to be no objection to following Vetter in comparing these 
muscles with the adductor muscle of the jaw, the origin of which 
has moved to the anterior face of its cartilage as the branchial 
adductors have tended to do in many other forms. 
As explained above there is some uncertainty as to the nerve 
supply of these muscles. It is easily shown that the nerve to 
each reaches it by crossing the posterior face of the cartilage. 
And so they do in Amia according to Allis (97). Vetter on the 
other hand, while sometimes difficult to interpret, nevertheless 
seems to imply the existence of other conditions in the fishes 
that he describes. He states (’78, p. 449) that in Chimaera the 
adductor muscles are supplied by the ‘R. post.’ of the correspond- 
ing inter-branchial branch of the vagus. If ‘R. post.’ means 
ramus posttrematicus vagi, as seems most probable, then it is 
the anterior nerve of each gill that supplies the muscular branch, 
and he leaves the first adductor muscle of Chimaera entirely 
unaccounted for, probably a mere oversight. If this interpreta- 
tion of his meaning be correct, it brings this statement into 
accord with what he says elsewhere (’74, p. 445) regarding the 
elasmobranchs, where the glossopharyngeal nerve is mentioned 
as supplying its appropriate adductor muscle, for, it will be re- 
called, the glossopharyngeal supplies the ramus posttrematicus of 
the front gill. His comment on Acipenser throws no light on the 
subject. Now if Vetter’s observations are accurate and his state- 
ments correctly interpreted it appears that the adductor muscles 
belong with the posttrematic nerve of their respective gills. If 
