VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 373 
from the adductor is much more in accord with the conditions 
in the Batoidei. 
In the Batoidei there is no inferior postspiracular ligament. 
In these fishes the proximal portion of the myotome of the 
hyal arch apparently passed over onto the anterior surface 
of the cartilaginous bar of the arch exactly as in the branchial 
arches, but, because of the marked change in the angle 
between the epihyal and ceratohyal (see Parker, 1876, PI. 61, 
fig. 4), and the separation of the epihyal from the pharyngo- 
hyal, which latter element was utilised to form the hyo- 
mandibula (Allis, 1915), the proximal portion of the myotome 
here separated from the distal portion throughout its entire 
length, and no small middle portion was cut out to form an 
adductor. The proximal portion then acquired attachment 
on the pharyngohyal (hyomandibula) ami gave rise to the 
musculi levator and depressor hyomandibularis of Tiesing’s 
(1895) descriptions, and possibly also to the depressor 
mandibularis, which is said by Tiesing to be innervated by 
the nervus facialis. The remaining, distal portion of the 
myotome formed the constrictor superficialis. No adductor 
muscle being differentiated in this arch in these fishes, an 
inferior postspiracular ligament was naturally never de- 
veloped. 
Ruge says that, in the Selachii, the dorsal and ventral 
portions of the hyal constrictor superficialis both Tend to 
separate into superficial and deeper layers, the former 
acquiring an insertion on the mandibular cartilages while the 
latter retains its primary insertion on the hyal cartilages. 
The insertion of certain of the fibres on the mandibular 
cartilages he considers to be an ancient acquisition of these 
fishes, and whenever it is wanting, in recent fishes, he con- 
siders it to be due to retrogression. The nervus hyoideo- 
mandibularis facialis is said to always lie external (anterior) 
to that part of the hyal constrictor that is inserted on the 
hyal cartilages, and to usually, but not always, lie internal 
(posterior) to the fibres inserted on the mandibular cartilages. 
In the region of the hyomandibulo-ceratohyal articulation, 
