VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 347 
to be noted that this partial fusion of these fibres with the 
trapezius is not found in my specimens of Scyllium, and it is 
to embryos of this particular fish that Edgeworth’s descrip- 
tions relate. This view of the development of the trapezius 
from the constrictor of the ultimate branchial arch is also in 
fullest accord with GreiTs very complete descriptions of its 
development in Ceratodus. 
In this latter fish, Ceratodus, a muscle called by Greil the 
dorsoclavicularis is said by that author (1913, p. 1343) to 
represent the phylogenetic beginning of a musculus tra- 
pezius. This dorsoclavicularis is said by Greil (loc. cit.,p. 1139) 
to be derived from a ventral process of the posterior half of 
the second trunk myotome, which, forking over the fifth bran- 
chial cleft, sends one process down anterior to that cleft and 
the other posterior to it. The former process lies in the fourth 
branchial arch, and from it is developed the axial mesoderm 
of that arch. The posterior process is shown in GreiTs fig. 1, 
plate 52, apparently lying posterior and partly internal to a 
branchial pouch which is called, in the index lettering, the 
u siebenten Schlundtasche,” that is, the sixth branchial pouch. 
In figs. 3 and 4 of the same plate the process is shown lying 
directly external to this sixth branchial pouch, close against 
the posterior edge of the fifth branchial cleft. The sixth 
branchial pouch never breaks through to the exterior. 
The posterior fork of the ventral process of the second 
trunk myotome of Ceratodus accordingly lies in the fifth 
branchial arch alone, or both in that arch and the region 
of a sixth branchial arch that never develops. The process 
is said to separate into superficial and deeper portions. The 
deeper portion grows inward, dorsal to the pericardium, and 
forms a muscle which, although lying ventral to the pharyn- 
geal cavity, is called the musculus dorsopharyngeus. This 
large muscle is the exact serial homologue of a smaller 
muscle, called the interbranchialis IV, which is developed 
from the ventral end of the axial mesoderm of the fourth 
branchial arch ; and both these muscles, lying dorsal to the 
pericardial cavity and the tr uncus arteriosus, are serial 
