VISCERAL ARCHES OE THE GNATHOSTOME FISHES. 389 
which run directly into each other and so form a complete 
circuit in which it is impossible to tell where the .one nerve 
ends and the other begins. In the General Summary it is 
further said (loc. cit., pp. 744-5) that the ramus maxillaris 
inferior trigemini probably innervates the musculus interman- 
dibularis and all, or a part, of the inferior division of the 
geniohyoideus, and that the ramus hyoideus facialis probably 
innervates the superior division of the geniohyoideus, and a 
part, at least, of the inferior division of that muscle. The 
innervation, in each case, is only given as probable, and 
there is no slightest suggestion of any part of either of 
the muscles being innervated, at the same time, by both the 
nerves. 
Mandibular Arch. 
Dohrn says (1885, p. 13) that, in selachian embryos, a 
muscle is developed from a myotome that comes from the 
mandibular arch (eines Muskelschlauches welcher vom Kiefer- 
bogen kommt), and that this muscle corresponds to the 
ventral portion of the constrictor superficialis of the hyal 
arch. There is, as already explained, some question as to 
whether Dorhn considered a part of this muscle to be the 
homologue of the coracobranchialis of the branchial arches, 
but it is certain that the myotome, said to come from the 
mandibular arch, must be the ventral end of the mandibular 
myotome, and that the muscle said to be developed from it 
must be that part of the musculus intermandibularis of the 
adult that is primarily, if not actually, innervated by the 
nervus trigeminus. Edgeworth (1911) says, as has already 
been fully explained and discussed, that the myotome of the 
maudibular arch only extends to the ventral edge of the 
musculus adductor mandibulas, and that the musculus inter- 
mandibularis is developed from the walls of the cephalic 
coelom. There is thus here marked difference of opinion. 
Accepting Dohrn* s observations as correct, and assuming 
that there was primarily a premandibular arch separated 
from the mandibular arch by a visceral cleft, there must 
