ANATOMY OF HETERODONTUS: ENDOSKELETON 461 
arch slants obliquely backward so that the pharyngobranchial is 
considerably behind the outer segments of the arch. 
The upper or pharyngobranchial segment is a triangular- 
shaped cartilage, the apex of which points ventro-laterally, and 
the broad base of which forms its dorso-median margin. It is 
not bound by pronounced ligament to either the pharyngo- 
branchial of the opposite side or to the spinal column, but it is 
held in place dorsally by connective tissue. A ligament is 
attached to the neck of the pharyngobranchial just above its 
union with the epibranchial segment. To a further considera- 
tion of this ligament we shall return. Here it may be said 
simply that it passes posteriorly to the head of the following 
epibranchial segment. 
The epi- and ceratobranchials are stout cartilages, the latter 
being considerably longer than the former. Near their joint 
both cartilages are hollowed out (not seen in fig. 10) so as to 
increase the angle between the two. The joint between them is 
simple, the articulating surfaces being held together by a con- 
nective tissue capsule. 
The epi- and ceratobranchial segments are of great importance 
to the area since they alone possess cartilaginous branchial 
rays for the support of the gill tissues (b.r., fig. 10). On the 
first arch fourteen such rays are usually present, five on the epi- 
and nine on the ceratobranchial segment; the first on the cerato- 
branchial is in all cases the longest of the series. 
The hypobranchial segment of the first arch (h-b./, fig. 11) 
is much smaller than any of the other segments. It is not con- 
nected with the hypobranchial of ‘the other side or with a median 
basibranchial cartilage, but remains as a rudimentary cartilage 
connecting the ventral ends of the first ceratobranchial with the 
cerato- and basihyal cartilages. 
Dorsally the segments of the second, third and fourth arches 
are essentially like the first, except that the pharyngobranchial 
segment of the fourth has fused with that of the fifth arch (fig. 14). 
Ventrally, these arches differ from the first principally in that 
their hypobranchial segments are well developed (h-b.2-4, fig. 
11). The hypobranchial segment in these is so arranged as to 
