HYOMANDIBULA OF THE GNATHOSTOME FISHES ont 
ventro-mesial to the vena jugularis inferior, separated from that 
vein by certain muscles of the region but still in protective rela- 
tion to it. The vein here lies between the base of the extrabran- 
chial and the base of the adjacent ventral ray of the branchial 
series, definitely separating the one from ‘the other; which must 
be a secondary adaptation if the extrabranchials are simply 
modified branchial rays. 
In both Heptanchus cinereus and Acanthias blainvillii the 
bases of the dorsal extrabranchials lie, as they do in Mustelus, 
against the lateral wall of the vena jugularis, but in a single speci- 
men of Cestracion that I have, the dorsal ends of the dorsal 
extrabranchials pass dorso-mesial to the vena jugularis and al- 
most reach the pharyngobranchials mesial to that vein, thus 
lying against the dorso-mesial surface of the vein. The rela- 
_ tions of the dorsal extrabranchials of Cestracion to the vena jugu- 
laris are thus similar to those of the ventral extrabranchials of 
Mustelus to the vena jugularis inferior. The enlarged bases of 
these dorsal extrabranchials of Cestracion all touch each other 
and are bound together by connective tissue, but they have not 
fused into a longitudinal bar such as Gegenbaur (’72) describes 
in his specimen of this fish. 
In Raia radiata I find the cartilages described by Foote (97) 
as dorsal extrabranchials much as she describes them, but they 
are less completely fused with the expanded outer ends of the 
branchial rays. Like Gaupp (’05, p. 897), I should have been in- 
clined to consider this cartilage simply as a plate resulting from 
the fusion of the outer ends of the branchial rays, were it not that 
the dorsal (proximal) end of the’ cartilage presents two points, 
one of which is bound by ligament to the epibranchial of its own 
arch and the other to the pharyngobranchial of the next poster- 
ior arch. This dorsal end of the cartilage lies, as ‘do the bases 
of the extrabranchials in the Selachii above described, against the 
lateral wall of the vena jugularis. The anterior cartilage of the 
series lies in the hyal arch. 
No other Plagiostomi or other fishes were examined in this 
connection, but in the dissections of the few fishes above referred 
to, the dorsal interarcual cartilages to which reference has several 
