690 
PROFESSOR T, J. PARKER OK THE 
latter is poured into the anterior trunk by the transverse commissures {com. 2), and, 
finally, the posterior efferent artery loses altogether its connexion with the aortic 
arch of its own branchiomere, and acquires a secondary one with the succeeding arch. 
From the above considerations one is led to look upon the connexion of the first 
(hyoidean) efferent artery ( Ef. hr. A. 1) with-the first epibranchial artery (Epbr. A. 1 ) 
as a secondary one, and it then becomes a matter of considerable interest to find in 
Mustelus antarcticus distinct remains of the dorsal portion of the hyoidean aortic 
arch, and of its connexion with the dorsal aorta. 
From the dorsal end of the first efferent branchial artery arises a large vessel, the 
posterior carotid artery {Post. car. A.). This trunk passes forwards and inwards, 
ventrad of the proximal end of the hyomandibular, to the ventral surface of the 
auditory capsule, and then through a foramen in the skull floor to the orbit. Its 
further course will be described hereafter (p. 695), the point of interest for the present 
purpose is that shortly before entering the foramen just mentioned—at the point x in 
fig. 6—it gives off a very slender vessel {y), which passes backwards and inwards along 
the ventral aspect of the skull and vertebral column, and joins with its fellow- of the 
opposite side to form a delicate longitudinal median trunk (z), w'hich is continued 
backwards to the junction of the first pair of epibranchial arteries. 
I think there can be no doubt that the posterior carotid artery, from its origin to 
the point x. together with its backward continuation y, represents the dorsal portion 
of the hyoidean aortic arch, or hyoidean epibranchial artery, the altered direction of 
the vessels being accounted for by the changed position of the hyoid arch. 
The median trunk z is, as obviously, the actual anterior portion, or what may 
be called the inter-hyoidean section, of the dorsal aorta. It has clearly nothing 
whatever to do with the arteria vertebralis impar of Myxinoids (17) which it 
resembles at first sight, since the latter is a secondary forward prolongation of the 
aorta altogether cephalad of the gills. As this anterior portion of the dorsal aorta 
undergoes complete atrophy—if indeed it ever exists—in the Rays as well as in 
Ilolocephali, it is a matter of some interest to find it persisting in a typical Selachian, 
and one is led to inquire whether it is actually absent in those forms the arteries 
of which have been described, or whether it has hitherto been overlooked. I can 
only say that I have failed to find any mention of it. 
The course of the mandibular aortic arch is by no means so clear and can only he 
decided by a detailed study of development. From about the middle of the first 
efferent branchial artery is given oft* the pseudobrancliial artery (fig. 6, Psbr. A.), 
which passes forwards external to the articulation of the hyomandibular and epihyal, 
and then extends forwards, upwards, and inwards to the spiracular pseudobranch 
(Psbr.), the rudiment of the mandibular gill of the embryo. The blood is collected 
from this organ by the anterior carotid artery {Ant. car. A.) which is continued 
inwards and forwards to the orbit, passes through a foramen in the side wall of 
the skull, and becomes the cerebral artery {Cereb. A, vide infra, p. 694) : while 
