HYOMANDIBULA OF THE GNATHOSTOME FISHES 573 
muscle-sheet were then to abort, the pharyngobranchials still 
retaining their attachment to the neural axis, the dorso-mesial 
ends of those cartilages would still lie dorsal both to the efferent 
branchial arteries and the dorsal aorta. But, if the attachment 
to the neural axis had not been acquired, or had been lost, 
those ends of the pharyngobranchials would lie ventral to the 
arteries, and morphologically ventral also to the dorsal aorta. 
In either case both the pharyngobranchials and the muscle- 
sheet, if it persisted, would lie definitely ventral to the vena 
jugularis, their relations to that vein not having been in any way 
disturbed. And this is what I find in the very unsatisfactory 
descriptions of other fishes that I have at my disposal. 
In Stegostoma, Luther (’09) says that the epibranchials of 
the first and second branchial arches come into contact with the 
neurocranium, ventro-mesial to the hyomandibular articulation, 
and hence, as will be later shown, certainly ventral to the vena 
jugularis. The relations to the lateral dorsal aorta are not given. 
In Ceratodus, Krawetz (10) says that the epibranchial of the 
second branchial arch often comes into contact with the auditory 
capsule, and reference to Greil’s (’13) figures will show that the 
point of contact must certainly lie dorso-lateral to the arteria 
carotis interna, which is the anterior prolongation of the lateral 
dorsal aorta, and ventral to the vena jugularis. In Amia, I 
(Allis, ’97, fig. 61, pl. 36) found the dorsal (proximal) end of the 
pharyngobranchial of the first branchial arch attached to the 
neurocranium dorsal to the common carotid artery (lateral dor- 
sal aorta) and ventral to the vena jugularis, the pharyngobran- 
chials of the other branchial arches all lying ventral to the aorta 
as well as to the vena jugularis. In Scomber (Allis, ’03) the dor- 
sal end of the pharyngobranchial of the first branchial arch is also 
in contact with and attached to the neurocranium dorsal to the 
lateral dorsal aorta and ventral to the vena jugularis, and, as the 
dorsal end of the first pharyngobranchial of certain of the Clu- 
peidae (Ridewood ’04) comes into contact with the neurocra- 
nium immediately ventral to the trigemino-facialis chamber, 
through which the vena jugularis undoubtedly passes as it does 
in Amia and Scomber, the pharyngobranchial must there have . 
