VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 399 
differentiated in the hyal arch, but indications of the begin- 
nings of its differentiation may there be found. 
In the hyal arch of the Selachii, an adductor muscle was 
probably developed exactly as in the branchial arches, but it 
was later transformed into the inferior postspiracular liga- 
ment. Arcualis and interarcualis muscles are not found in 
this arch of the adult, but Dolirn says that they are repre- 
sented, in embryos, by ligaments, which he does not, however, 
describe. The coracobranchialis of Dohrn’s descriptions of 
the branchial arches is not differentiated in this arch. The 
dorsal and ventral portions of the constrictor tend to separate 
into deeper and superficial layers, as Huge has stated, the 
deeper layer retaining its primitive insertion on the carti- 
laginous bar of its arch, while the superficial layer acquires 
a secondary insertion on the cartilaginous bar of the 
mandibular arch. 
In the Batoidei, an adductor muscle was not differentiated 
in the hyal arch, and there is accordingly, in these fishes, no 
inferior postspiracular ligament. The proximal (anterior) 
portion of the primitive constrictor of this arch is differenti- 
ated into the levator and depressor hyomandibularis, and 
probably also the depressor mandibulae, these muscles 
replacing the adductor, arcualis, interarcualis, and Dohrn’s 
coracobranchialis of the Selachii. The so-called septa in 
these fishes are probably similar to those in the Selachii, 
but this cannot be definitely determined from existing 
descriptions. 
In the mandibular arch of the Selachii, a small adductor 
muscle was probably developed exactly as in the more 
posterior arches. Later, because of the suppression of the 
branchial diaphragm related to this arch, excepting as it may 
be represented in the anterior wall of the spiracular canal, 
the entire constrictor muscle was forced, in its middle portion, 
-onto the anterior surface of the cartilaginous bar of its 
arch, and, acquiring insertion on the palatoquadrate and 
mandibula, where it crossed their lateral edges, Avas added to 
the small, pre-existing adductor, and so gave rise to the 
