464 J. FRANK DANIEL 
young. The upper segment (ep.br.6) is a slender cartilage, 
(loosely) attached dorsally to the fifth pharyngobranchial, but 
ventrally, as is seen from the figure, it is fused solidly to the 
fifth epibranchial segment. If viewed from the median side, 
however, it is seen to be continuous with the lower segment. 
I have already described for the first branchial arch a ligament 
which extends from the neck of the pharyngobranchial backward 
and downward to be attached to the epibranchial following. 
To this I shall now return with a hope that it may be of service 
in interpreting the dorsal piece in question. It so happens that 
this ligament forms the floor of a pit, the anterior wall and the 
roof of which are formed by the neck of the first pharyngo- 
branchial and the posterior wall by the upper part of the follow- 
ing epibranchial segment.2. Similar pits follow the second, third 
and fourth (lg., text fig. B) arches. Now, following the fifth 
pharyngobranchial there is also a pit (pt., text-fig. B) which, in 
its surroundings, is identical with the first. From the neck of 
the fifth pharyngobranchial the ligament passes under the pit, 
giving some of its fibers to the posterior wall; other fibers from 
the fifth pit continue across with the ligament from the fourth 
pit to join the pectoral girdle. The piece forming the posterior 
wall (ep.br.6) I therefore interpret as the sixth epibranchial 
segment. 
The lower or second segment, the ceratobranchial, is displaced 
forward so as to lie laterad of the joint of the fifth arch. This 
displacement in Heterodontus is due to the enlargement and 
crowding forward of the massive pectoral girdle. 
There is present in the anterior wall of the spiracle a thin 
spiracular cartilage (sp.c., pl. 3, and fig. 12) which, like bran- 
chial rays, supports a septum for gill filaments. This cartilage 
is (generally) interpreted as a fused series of cartilaginous 
rays which originally belonged to the palato-quadrate segment. 
Because of the outward flaring of the quadrate the spiracular 
cartilage comes to be widely separated from the quadrate. 
* This ligament probably represents the median interarcuales muscles. 
