574 EDWARD PHELPS ALLIS, JR. 
the same relations to the vein and artery that it has in Scomber. 
In Ammocoetes the dorsal ends of the branchial bars have fused 
with the neural axis, and Favaro (’08) shows the dorsal aorta 
lying ventral to them, and the vena jugularis and its anterior 
prolongations—the venae jugularis dorsalis and capitis lateralis 
—dorsal to them; and I find similar relations in a single specimen 
of Petromyzon that I have had examined. These so-called exter- 
nal cartilaginous arches of these fishes thus have the same rela- 
tions to these two important blood vessels that the so-called inner 
cartilaginous arches of the gnathostome fishes have. 
These relations of the so-called inner cartilaginous bars of the 
branchial arches to the aorta and vena jugularis are thus quite 
unquestionably not only a common feature but also a fundi- 
mental characteristic of all fishes, and, as it seems unquestionable 
that the prebranchial arches of the gnathostome fishes were pri- 
marily similar to the branchial ones, they must have been a pri- 
mary characteristic of those arches also. These relations have, 
in fact, persisted in both the hyal and mandibular arches of all 
the Plagiostomi, so far as I can find described and as will be 
later shown, and because of this, and also because all apparent 
deviations from the rule in the prebranchial arches: of other gna- 
thostome fishes can be fully explained by the assumption of the 
association, or fusion, with the inner cartilaginous bars of those 
arches, of the related extrabranchial or interarcual cartilages, I 
assume, as stated in the opening paragraph of this paper, that 
they have persisted and are invariable in all gnathostome fishes 
in so far at least as the vena jugularis is concerned, which is the 
important consideration in this discussion. But, before con- 
sidering these prebranchial arches, the extrabranchial and inter- 
arcual cartilages of the branchial region must be considered. 
The extrabranchials are generally considered to be peculiar to 
the Plagiostomi and to be specially modified dorsal and ventral 
ones of the branchial rays of that arch in the diaphragm of which 
they lie (K. Furbringer ’03). They are said to differ radically 
from the other branchial rays in being attached, respectively, to 
the pharyngeal and hypal, instead of to the epal and ceratal ele- 
ments of the inner arches, and this attachment is to those ele- 
