VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 
393 
morphologically dorsal end, of the musculus intermandibu- 
laris, the constrictor fibres in these fishes thus apparently 
having retained their full primitive lengths. 
Luther (1909, p. 49) considers the levator labii superioris 
(praeorbitalis) to have been primarily simply a bundle of the 
adductor mandibulae that had it£ origin at a high level on the 
neurocranium, anterior to the eyeball. The more ventral 
origin of this muscle, from the antorbital process, found in 
Chlamy doselachus and certain other Plagiostomi, he considers 
to be secondary and correlated either to an enlarged eyeball 
or to a large gape of the mouth with the angle of the gape far 
posterior, the muscle here secondarily becoming a “ Spreizer ” 
of the articulating ends of the upper and lower jaws. The 
eyeball is considered by him (loc. cit., p. 36) to have been 
the chief one of these two causes of the splitting off of this 
bundle from the remainder of the adductor mandibulae, and 
if this be so, the eyeball thus being assumed to have lain in 
the path of the muscle-fibres of the arch as they pushed dor- 
sally to acquire insertion on the neurocranium, it would seem 
as if this split in the muscle must have begun at the dorsal 
end of the primitive constrictor and not at the dorsal end of 
that middle portion of that muscle that is usually considered 
to, alone, have given origin to the adductor mandibulae. The 
dorsal portion of the praeorbitalis would then contain the 
anterior fibres of the dorsal muscle, Csd 2 , of Vetter’s descrip- 
tions, and where the praeorbitalis extends beyond the angle of 
the ^ape the split that separates it from the adductor would 
extend from the dorsal end of the constrictor as far at least 
as the ventral end of the adductor. Such an extensive split 
in this myotome can certainly not be explained simply by the 
eyeball having caused the dorsal fibres of the constrictor to 
diverge anteriorly and posteriorly in order to acquire a dorsal 
attachment on the neurocranium, and if Luther is correct in 
his conclusion that this muscle had primarily its origin at a 
high level on the neurocranium, a much more rational expla- 
nation would seem to be that this muscle belongs to a pre- 
mandibular arch. The recorded innervation of the muscle. 
