ANATOMY OF HETERODONTUS: ENDOSKELETON 457 
rior tip of which extends considerably laterad of the anterior. In- 
side of and below the teeth, there is a long ridge (md.r., fig. 7) 
from which a tendinous bridge passes to a similar ridge on the 
other side; to the lower sides of this ridge the strong coraco- 
mandibularis muscle is attached; near the quadrato-mandibularis 
joint and mediad there is present a prominent mandibular knob 
(kb.) against which the second arch abuts. 
The joint between the palato-quadrate and the mandible, 
like that in Chlamydoselachus (Goodey ’10, pp. 544-545) and 
Heptanchus (Gadow ’88, pp. 452-453) forms a double ball and 
socket. The anterior articulation is formed by a ball of the 
mandible fitting into a socket of the quadrate. The posterior 
articulation consists of a large socket in the outer angular part 
of the mandible, into which a ball from the hyal process of the 
quadrate fits. Between the two articulations in Heterodontus 
is a space, somewhat like that described by Gadow (’88) for 
Heptanchus. 
A description of the articulations of the first arch, further 
than the attachment of the palato-quadrate to the cranium 
as above described (p. 456), may be deferred until a study is 
made of the second or hyoid arch. 
The hyoid arch (fig. 6) as we have seen, is also composed of 
two segments. The upper division, the epihyoid, becomes in 
Heterodontus an important suspensorium for the mandibular 
or first arch; it is called the hyomandibula (hm.). The lower 
segment of the arch is the ceratohyoid or hyoid proper (c-h.). 
Connecting the two ceratohyoids of opposite sides is a median 
unpaired piece, the basihyal cartilage (b.h., fig. 11). 
Both of the segments of the hyoid arch are heavy cartilages. 
The hyomandibula is thickened both proximally, where it fits 
into the deep fossa under the auditory capsule, and distally, 
where it joins the ceratohyoid and touches the mandible near 
the quadrato-mandibular joint. The ceratohyoid is consider- 
ably longer than the hyomandibular segment, and extends 
forward and inward to meet the basihyal. 
Articulations of the hyoid arch. The hyomandibula is bound 
by a strong capsular ligament to the hyomandibular fossa in the 
