VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 319 
the ventral portion of each constrictor superficialis, is probably, 
as already stated, what misled Dohrn in his interpretation of 
the muscles in embryos of Scy Ilium. 
Musculi interbranchiales, not found in Heptanchus, have 
been differentiated in Acanthias. They are said to be found 
in each of the first four branchial arches of the fish, but not 
in the hyal arch. The muscle is said to form a thin muscle- 
sheet which extends, in each arch, between the extrabranchials 
and the inner cartilaginous bar of the arch, completely filling 
the space between them. Marion says that “they are in no 
sense superficial, nor circular in the same sense that the other 
muscles are, and they lie in a different plane.” Huge (1897, 
p. 219) says that they extend from the branchial bar of the 
arch to the free edge of the related branchial diaphragm, and 
form the middle part of a “ Muskel-Scheidewand,” thus lying 
between dorsal and ventral portions of the muscles of the 
arch and wholly separating them from each other. Ruge’s 
conception of these muscles is thus totally different from 
Vetter’s and Marion’s, but it agrees with DohnTs description 
of the muscles in embryos. Vetter says that the interbran- 
chialis of each arch lies close against the anterior (external) 
surface of the branchial rays of the arch, and extends to the 
outer ends of those rays, there passing insensibly, without 
definite boundary, into that part of the constrictor superficialis 
of the arch that traverses the branchial diaphragm. The 
distal and larger part of the fibres of each interbranchialis 
are said to arise, both dorsally and ventrally, in part from 
the extrabranchial of the related arch and in part from the 
linear aponeurosis that overlies that extrabranchial, while 
the proximal (anterior) fibres arise, both dorsally and ventrally, 
in part from a feeble ligament that extends from the extra- 
branchial of the arch to the extrabranchial of the next anterior 
arch, and in part from the latter extrabranchial. 
In Mustelus, the proximal fibres of the constrictor super- 
ficialis of each branchial arch are said by Tiesing (1895) to 
arise from a dorsal fascia similar to that described by Vetter 
in Heptanchus, while the distal (posterior) fibres arise, as in 
