or 
HYOMANDIBULA OF THE GNATHOSTOME FISHES 58 
the neurocranium. No association of these cartilages with the 
muscles or ligaments of the region is mentioned by Luther, but 
_ Gadow (’88) describes ligaments similar to those found in Chlamy- 
doselachus, but no cartilages, in Heptanchus, Oxyrhina and 
Sphyrna. In each of these fishes two ligaments are said to have 
their origin from the dorsal end of the hyomandibula and, run- 
ning posteriorly, to have their attachments, one to the dorsal 
end of the ceratobranchial of the first branchial arch and the 
other to the epibranchial of the same arch at about the middle of 
its length; but it must be that it is the epibranchial and pharyn- 
gobranchial of the arch, and not the ceratobranchial and epi- 
branchial, that are here respectively meant. 
From Schauinsland’s, Luther’s and my own observations, it 
‘thus seems certain that the hyomandibula of the Selachii is a 
simple epal element of the hyal arch, and the reason why this 
element, instead of the pharyngohyal, articulates with the neuro- 
cranium is found: in the attachment of the dorsal ends of the 
cartilaginous bars of the hyal and branchial arches beneath the 
vertebral column and hence posterior to the neurocranium; in 
the sigma-form which has been impressed, for some unknown 
reason, on the cartilaginous bars of these arches of these fishes; 
and in the fact that the pharyngeal elements of these bars appar- 
ently always articulate, not with the dorsal (proximal) ends of 
the epal elements, but with the posterior edge of the dorsal ends 
of those elements, leaving the dorsal ends themselves exposed 
(see Gegenbaur’s figures). Because of these several conditions, 
when the auditory capsule began to develop, it came into con- 
tact with the sigma-shaped cartilaginous bar of the hyal arch at 
the dorsal bend in that bar, and when articulation with the 
neurocranium was formed it was necessarily with the exposed 
dorsal end of the epihyal. The pharyngeal element, impeded in 
its development by the bulging auditory capsule, then underwent 
reduction. The nerves and blood vessels of the region, with a 
single exception, retained unchanged their topographical rela- 
tions to the epihyal (hyomandibula), if the conditions that I 
find in Chlamydoselachus, Heptanchus and Mustelus are typi- 
cal of all the Selachii; for in these three fishes the lateral dorsal 
