VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 311 
the trapezius each myotome forms a transversely broad plate 
in the branchial septum. The part internal to the branchial 
bar forms the adductor; the part external to the bar forms 
dorsally the arcualis dorsalis, and below that the inter- 
branchial, whilst the external edge forms the constrictor 
superficialis.” The constrictor superficialis was accordingly 
developed from that part only of the myotome that lay 
primarily in the branchial diaphragm, and although a later, 
ventral downgrowth of this muscle is said to take place, as 
just above stated, no mention is made of any dorsal upgrowth 
from that part of the myotome that lies ventral to the trapezius. 
It is even definitely said (loc. cit., 251) that no levator 
muscles are developed in the branchial arches of this fish. 
Here the embryological conditions, thus described, do not 
agree with the conditions found in a 42 cm. specimen of this 
fish that I have examined, and which will be later fully 
described, for in each branchial arch of this fish there is, as 
in Heptanchus, a dorsal portion of the constrictor superficialis 
which extends beyond the dorsal extrabranchial and overlaps 
externally the musculus trapezius. In the hyal arch of 
embryos of this fish Edgeworth himself describes this dorsal 
prolongation of the constrictor, and he there not only calls it 
the levator hyoidei, but says (loc. cit., p. 228) that it is 
serially homologous with the levator muscles of the branchial 
arches of the Teleostei. The levator hyoidei is a part of the 
einuscl Csd 2 of Tetter’s descriptions of the Selachii described 
by him, and as this muscle, in Heptanchus, and in my 
specimen of Scyllium, certainly has its serial homologues in 
the dorsal ends of the constrictores of the branchial arches, 
if the one is the homolpgue of the levators of the Teleostei 
the others must evidently also be. 
The term “ arcualis dorsalis” is said by Edgeworth (loc. 
cit., p. 226) to be employed by him, as proposed by 
Furbringer, to designate the iuterarcuales dorsalis II and III 
of Tetter’s descriptions, one of which muscles is, however, 
(Tetter, Furbringer) an interarcual and not an arcual muscle. 
The interarcualis dorsalis I of Tetter’s description is called 
