586 EDWARD PHELPS ALLIS, JR. 
aorta still lies mesial and hence ventral to the epihyal (hyoman- 
dibula), the vena jugularis lies dorsal to that element, the com- 
missural vessel that forms the definitive afferent pseudobranchial 
artery (Allis 711 b, ’12) crosses its external surface, and the ner- 
vus hyomandibularis facialis runs outward across its anterior 
edge to reach its external (dorso-lateral) surface, thus lying mor- 
phologically dorso-external both to it and to the pharyngohyal. 
The single exception to the rule is the efferent hyal artery. The 
efferent arteries of the branchial arches of these several fishes all 
cross the external surface of the distal end of the related pharyn- 
gobranchial in order to reach the external surface of the epibran- 
chial. In the hyal arch the artery. doubtless had primarily a 
similar course, but when that arch acquired articulation with the 
neurocranium by its exposed proximal end (which projected an- 
teriorly beyond the efferent artery of the arch) the artery lay 
posterior to the articulating surfaces and hence passed over the 
posterior edge of the epihyal to reach its external surface. 
The hyomandibula of the Selachii is, so far as I can determine 
from the literature and material at my disposal, always attached 
to the neurocranium by one or two more or less important liga- 
ments in addition to the special articular ligaments. Ridewood 
(95) describes both of these ligaments in Sceyllium and calls 
them the superior and inferior postspiracular hgaments. The 
superior ligament is said by him to have its attachment on the 
auditory capsule dorsal to the vena jugularis (‘‘vein connecting 
the orbital and anterior cardinal blood sinuses’’) and to be at- 
tached, ventrally, partly on the palatoquadrate and partly on 
the outer surface of the lower end of the hyomandibula. The 
nervus hyomandibularis facialis runs outward between the liga- 
ment and the hyomandibula, posterior to the one and anterior to 
the other. The inferior ligament is said to be attached by one 
end to a lateral projection at the base of the skull, below and 
behind the foramen for the nervus trigeminus, and by the other 
end to the postero-internal edge of the lower half of the hyoman- 
dibula, and partly also to the upper end of the ceratohyal. 
Gegenbaur (’72) describes and figures the inferior postspiracu- 
lar ligament in Heptanchus (1’, fig. 1, pl. 15), without so naming 
