HYOMANDIBULA OF THE GNATHOSTOME FISHES 52D 
ments of the arch next posterior to the one in the diaphragm of 
which they lie instead of to the elements of their own arch. 
Dohrn (’84, p. 119) says that they never articulate with the ele- 
ments to which they are attached, which doubtless means that 
their bases do not reach and, come into contact with those ele- 
ments, but Fiirbringer (1. ¢., p. 428) calls attention to the fact 
that, while this is true of most of the Plagiostomi, it is not true 
of Laemargus. Fiirbringer does not say with which element of 
the arch the extrabranchial of Laemargus comes into contact, 
but Gegenbaur’s (’98, fig. 270) reproduction of White’s figure of 
a branchial arch of this fish shows the contact with the dorsal 
end of the epibranchial. But however this may be, the impor- 
tant consideration in this discussion is that the conditions in Lae- 
margus definitely show that the extrabranchials can come into 
contact with some element of the inner cartilaginous bar of the 
arch next posterior to the one to,which they belong; and if they 
be modified branchial rays, as is generally considered to be the 
case, it is evident that they have potentially the possibility of 
coming into contact with some element of their own arch and of 
there fusing with it, as the branchial rays of Torpedo actually 
do. According as an extrabranchial is attached to an element 
of its own arch or to one of the next posterior arch, its relations 
to the nerve and artery of the arch to which it is so attached 
change from posterior and superior in the former case to anterior 
and inferior in the latter, this at once suggesting the suprapharyn- 
gobranchials and infrapharyngobranchials of van Wijhe’s (’82) 
descriptions of ganoids and Polypterus; and it was the evident 
suggestion of the extrabranchials representing one or the other of 
these two elements, both of which were considered by van Wijhe 
to belong to and form part of the inner cartilaginous bar of a 
complete and normal branchial arch, that led me to undertake 
the present study. And it is perhaps needless to say that had I 
known how far it would lead me I should not have undertaken 
it with the limited material at my disposal. 
The extrabranchials are said to be developed either in sup- 
porting and protecting relation to the branchial diaphragms, as 
are the other branchial rays (Dohrn ’84), or as a special adapta- 
