VISCERAL ARCHES OF THE G N AT H 0 s TO M E FISHES. 375 
latter cartilage. The fibres of the hyal constrictor, all 
primarily inserted on the hyomandibula, were then overlapped 
externally by the palatoquadrate, and the dorso-posterior 
edge of the latter cartilage lay posterior (distal) to the 
nervns facialis. The superficial fibres of the hyal constrictor 
then acquired insertion on the palatoquadrate along the line 
where the dorso-posterior edge of that cartilage crossed them, 
and so acquired a position external to the nervus facialis. 
Other, deeper fibres of the muscle then followed and joined 
the superficial ones. Where the dorsal end of the hyo- 
mandibula lay at a considerable distance from the palato- 
quadrate, the fibres of the constrictor simply pushed bodily 
forward, carrying the nervus facialis with them, and so 
retained their primitive position internal (posterior) to that 
nerve. 
In the Holostei and Teleostei the conditions are here quite 
different from those in the Selachii. In the former fishes, 
the epihyal does not acquire articulation with the neuro- 
cranium; the posterior articular head of the hyomandibula 
quite certainly being formed by the fusion of the supra- 
pharyngobranchial of the arch, derived from the basal 
portion of the extrabranchial of the arch, with the epihyal 
(Allis, 1915). That part of the constrictor superficialis that 
lay dorsal to the suprapharyngobranchial (extrabranchial) 
must then have been cut off from the ventral portion of the 
constrictor, and, lying between the suprapharyngobranchial 
and the cranial wall, it became modified to form the inusculi 
adductor hyomandibularis and levator and adductor operculi. 
These three muscles of the Holostei and Teleostei are, 
accordingly, together, the serial homologue of the levatores 
arcuum branchalium in their own branchial arches, and 
the homologue of the dorsal portion of the hyal constrictor 
superficialis of the Selachii. The branch of the nervus 
facialis that innervated these hyal muscles, lying primarily on 
the anterior (external) surface of the constrictor of the arch, 
would naturally have followed the muscles, and so come to 
lie internal to the dorsal end of the hyomandibula. The 
