VISCERAL ARCHES OF THU GNATHOSTOME FISHES. 349 
tion, and Edgeworth (loc.cit., p. 176) also questions it, but 
as it is a question that relates primarily to the origin of the 
mesoderm of these two arches, and involves that in the other 
visceral arches also, the question of a secondary change in 
the innervation of a muscle, as I am at present considering 
it, is not involved. 
Edgeworth (loc.cit., p. 243), in his own investigations, 
finds the trapezius of Ceratodus “proliferated from the 
outer side of the fifth levator ” arcus branchialis ; these two 
muscles together thus strikingly recalling the trapezius of 
the adult selachian. This origin of the muscle is thus in 
accord with my contention that it is developed from the 
primitive constrictor of the ultimate branchial arch of the 
fish, and from that constrictor only, and that it accordingly 
retains, in the adult, its normal and primitive innervation. 
In the Tejeostomi the conditions are probably strictly 
comparable to those in Ceratodus, but this cannot be 
definitely established from the descriptions given. Edge- 
worth says that the trapezius is developed, in all these 
fishes, from the fourth levator arcus branchialis. In Amia 
it is said (loc.cit., p. 239) to be represented by the fifth 
external levator of my descriptions of that fish, a muscle that 
I found innervated (Allis, 1897, p. 696) by a nerve that 
arose either from the base of the post-trematic branch of the 
third vagus nerve (nerve of the fourth branchial arch), or 
from the main trunk of the vagus near the base of that vagus 
nerve. In Acipenser the muscle is said by Edgeworth (loc. 
c i t . , p. 236) to be found in 8^- mm, embryos* given off from the 
fourth levator, but to be in process of disappearing in 11 mm. 
embryos. In the adult it is said, on Vetter’s authority, to be 
absent. The fifth levator of Vetter’s descriptions of this fish 
is said to be developed from the fifth branchial myotome, and 
although it persists in the adult, it is not considered by 
Edgeworth to represent the trapezius, as it does in Amia. 
This seems singular, for in Polyodon there is a well developed 
trapezius (Danforth, 1913, p. 141), innervated by a branch 
of the vagus, and but four levatores arcuum branchialium ; 
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