HYOMANDIBULA OF THE GNATHOSTOME FISHES 593 
element of the arch that thus came into supporting relations to 
the posterior wall of the spiracular cleft may explain the absence 
of branchiae on that wall of the cleft in these fishes, but this 
would not apply to the Selachii. In these latter fishes the ab- 
sence of these branchiae may be correlated to the interposition 
of the superior postspiracular ligament between the cleft and 
the epihyal. 
If this be the manner in which the cartilages of this arch have 
been developed, and there seems every reason to believe that it 
is, there can be no possible question of these cartilages repre- 
senting two visceral arches, for the double concentration of meso- 
derm cells said by Dohrn (’85) to take place in this arch in em- 
bryos of these fishes would be fully accounted for; one center 
normally representing and giving rise to the pharyngohyal and 
the other to the remainder of the arch. There is also, under 
my assumption, no need to assume either that an important por- 
tion of the cartilaginous bar of a visceral arch has completely 
aborted in this region (van Wijhe, ’01), or that a ventral segment 
(ceratohyal) of the arch has, by secondary segmentation, fur- 
nished a complete duplicate set of dorsal segments in its arch 
(Gegenbaur); and the break in the arch caused by the assumed 
change in position of the pharyngohyal would probably account 
for the difference in the muscles of the region as compared with 
selachians, a subject I have not yet been able properly to con- 
sider. 
In the Holocephali, Hubrecht (’77) shows two cartilages in 
the dorsal half of the hyal arch, the smaller, dorsal one project- 
ing postero-mesially, as the pharyngeal elements of the hyal 
and branchial arches do in the Selachii. Schauinsland (’03) 
shows a similar arrangement in an embryo of Callorhynchus, and, 
as already stated, he identifies the two cartilages as the epihyal 
and pharyngohyal, and says that the epihyal is certainly the 
homologue of the hyomandibula of other selachians (sicherlich 
homolog dem Hyomandibulare der Uiibrigen Selachier). This, 
if I am right in my conclusions, is correct in so far as the Selachi 
are concerned, but incorrect for the Batoidei. In a specimen of 
Chimaera colliei which I have examined, neither the epihyal nor 
