372 
EDWARD PHELPS ALLIS. 
some indication of the piece so cut out of the hyal muscle 
should be found in some stage of development of these fishes. 
According to Dohrn, it is not found in embryos, and he 
further says that the conditions there are such as to preclude 
the possibility of its development. There is, however, in the 
adults of these fishes a large and important ligament, the 
inferior postspiracular ligament, found in the hyal arch but 
not in the branchial arches, and not accounted for in Dohrn’ s 
descriptions of embryos. 
In a recent work I suggested (Allis, 1915) that this inferior 
postspiracular ligament of the Selachii was probably derived 
from the musculus arcualis dorsalis of the hyal arch. I at 
that time accepted the currently expressed opinion that an 
adductor muscle was not differentiated in this arch, or that if 
differentiated it had later completely aborted. My present 
work leads me to doubt both these assumptions, and it now 
seems to me much more probable that the ligament is derived 
from the adductor of the arch than from the arcualis dorsalis. 
The ligament is found in nearly all, if not in all, the Selachii, 
and it is not found either in the Batoidei or the Teleostomi. 
In the Teleostomi the plagiostoman adductores are not found 
even in the branchial arches, as has been already fully 
explained, this accounting for the absence of the ligament in 
the hyal arch of these fishes; and the reason for its absence 
in the Batoidei will be considered immediately below. In the 
Selachii, the adductor, probably developed exactly as in the 
branchial arches, ceased to be of functional value, doubtless 
because of the intimate attachment of the cartilages of the 
arch to those of the mandibular arch, and, travelling upward 
along the epihyal until it reached and acquired insertion on 
the chondrocraniuin, it became the inferior postspiracular 
ligament. The relations of the ligament to the nerves, 
arteries, and veins of the region, said by me to be in accord 
with the derivation of the ligament from the arcualis dorsalis 
of the arch, are equally in accord with its derivation from the 
adductor of the arch, and, while the ligament might appa- 
rently have been developed from either muscle, the derivation 
