HYOMANDIBULA’* OF THE GNATHOSTOME FISHES 619 
SUMMARY 
In all fishes, so far as I can find from the literature and mate- 
rial at my disposal, the pharyngeal elements of the branchial 
arches always le ventral to the vena jugularis and also pri- 
marily ventral to the dorsal aorta. But whenever these pharyn- 
‘geal elements or the epal elements of the branchial arches come 
into contact with the neurocranium or the vertebral column, the 
point of contact, although still remaining ventral to the vena 
jugularis is then always lateral, and hence actually dorsal, to 
the dorsal aorta or its anterior prolongations, the lateral dorsal 
aorta of either side. 
In all the Elasmobranchii, whenever the primary dorsal ends 
of the cartilaginous bars of the prebranchial arches come into 
contact with the neurocranium the point of contact is also always 
ventral to the vena jugularis and dorsal to the lateral dorsal 
aorta; and this is apparently also the relation of these blood ves- 
sels to the corresponding arches in Ammocoetes. 
This would accordingly seem to be such a fundimental charac- 
teristic of the visceral arches of all craniate fishes that it is a 
legitimate conclusion that the dorsal ends of the inner cartilag- 
inous bars of all of these arches, in all vertebrates, primarily 
lay ventral to the homologue of the vena jugularis of fishes, and 
that, when parts of the definitive cartilaginous bars of any of 
the visceral arches of any vertebrate articulate or fuse with the 
neurocranium dorsal to the homologue of that vein, those parts 
are primarily independent cartilages that have fused with the 
inner cartilaginous bars; or if it be that they are specially devel- 
oped processes of those bars, those processes do not represent 
the primary dorsal ends of either the pharyngeal or epal ele- 
ments of the inner bars. 
Dorsal and ventral so-called extrabranchial cartilages are 
found more or less developed in the branchial and hyal arches in 
all of the Plagiostomi, and their bases have acquired protective 
relations to the related vena jugularis. In fishes other than the 
Plagiostomi these extrabranchial cartilages have never been de- 
scribed as such, so far as I can find, but the dorsal extrabranchials 
