VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 367 
Two narrow and more or less extensive incisures were thus 
made in the muscle, and that part of the muscle that lay 
between these two incisures became the musculus inter- 
branchialis. Where the muscle fibres were not thus cut 
through, or did not become tendinous, the musculus inter- 
branchialis was simply a thinner portion of the primitive and 
continuous constrictor. That portion of the muscle that lay 
distal to the extrabranchials remained intact, and formed the 
continuous dorso-ventral fibres of the constrictor superficialis. 
The dorsal and ventral ends of the constrictores had, in the 
meantime, and in certain fishes, turned posteriorly, possibly 
influenced by the sigma form of the branchial bars. The 
arcual and interarcual muscles were then differentiated, and 
this, together with the overlappings and fusions of the dorsal 
and ventral portions of the constrictores superficiales with 
each other and with the musculus trapezius, and the forma- 
tion of tendinous aponeuroses where the muscle fibres 
crossed the underlying extrabrancliials, produced the many 
variations found in the adult. The constrictor of the ultimate 
branchial arch was utilised to form the musculus trapezius. 
In the Teleostomi, the straight form of arch was retained, 
and correlated to this the constrictores superficiales did not 
slip over the anterior edges of the related arches, but, in 
•certain fishes, certain of them slipped over the posterior edge 
of the related arch aud gave rise to the ganoidean adductors. 
The constrictor of each branchial arch then apparently became 
rudimentary in the middle of its length, doubtless because of 
modifications in the branchial lamellae and the development 
-of supporting branchial rods, but it was in part utilised to 
form the delicate radial muscles related to the supporting 
branchial rods. The dorsal and ventral portions of the primi- 
tive constrictor then became the levatores, and the transversi 
and obliqui dorsales and ventrales, and the ventral portion of 
the constrictor of the ultimate arch became the coraco- 
branchiales, or their homologues the pharyngoclaviculares. 
The levator of the ultimate branchial arch became, in certain 
•of these fishes, a musculus trapezius. 
