VISCKKAIi ARCHES OF THE GNATHOSTOM E FISHES. 397 
In the Teleostomi, the four typical elements of each 
branchial bar of recent fishes lie, approximately, in a single 
plane, and this must have been their primitive relation to 
each other. Primarily this plane must have been transverse 
to the axis of the body, but it later became inclined to that 
axis. Associated with this form of arch the branchial fila- 
ments of the gill- bearing arches are supported by cartilaginous 
or osseous rods. In the hyal arch there are, in addition, 
osseous branchiostegal rays which lie anterior to the modified 
constrictor of the arch. 
In the Plagiostomi, the dorsal and ventral elements of 
each branchial bar are directed postero-mesially at a marked 
angle to the middle elements of the bar, these latter elements 
lying, as in the Teleostomi, in a plane inclined to the axis of 
the body. A sigma form of bar is thus produced, and asso- 
ciated with it there are cartilaginous branchial rays in all the 
gill-bearing arches. These cartilaginous rays all lie, primarily, 
posterior to the constrictor muscle, of the related arch, bub 
the muscle fibres may later become in part inserted on them. 
In the Holocephali and Dipneusti, the dorsal elements of 
the branchial bars are directed postero-mesially, as they are in 
the Plagiostomi, while the ventral elements are directed 
antero-mesially, as in the Teleostomi. In the Holocephali 
there are, according to Vetter, cartilaginous rays both in the 
hyal and the branchial arches, and the visceral muscles as 
described by him seem plagiostoman in character. In the 
Dipneusti there are cartilages in the hyal arch that are con- 
sidered by Fiirbringer to be branchial rays, but there are 
neither branchial- rays nor supporting rods to the branchial 
filaments in the branchial arches ; and the branchial muscles 
are teleostoman in character. 
The constrictor muscle is found in a more primitive con- 
dition in the Selachii than in any others of the gnathostome 
fishes. Because of the sigma form of branchial bar in these 
fishes, the proximal (anterior) edge of the constrictor of each 
branchial arch has slipped forward over the anterior edge of 
the middle, posteriorly-directed angle of the sigma, and 
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