THE ELASMOBRANCII FISHES 63 



The union of tlie right and k^ft cartilages may be loosely made (notidanids) or 

 it may be firmer as in most forms; that of the mandibular cartilage, while firm 

 in the rays, is loose in most sharks except Squatina and Heterodontus. 



The articulation of the upper and lower jaws may be by a joint in all re- 

 spects similar to that of Heptanchus; such is the articulation in Chlai)ijjdosel- 

 achiis and Hderodontus. Or the joint may be slightly more specialized as 

 in Scyllimn. A highly specialized type found in the Elasmobranclis is that in 

 Raja (fig. 69), where a single ball and socket joint prevails. The joint may ))e 

 bound simply by short ligaments, or it may be firmly held by a most complex 

 ligamentous arrangement, as in Heterodontiis francisci (figs. 67 and 128) . 



Following the mandibular arch, and above the quadrate segment, is the 

 spiracular cartilage (sp.c, figs. 67 and 69) which supports the filaments in the 

 anterior wall of the spiracle, where such exist. This cartilage may be composed 

 of several segments (three in Cenirophorus, two in Acanthias) , or it may con- 

 sist simply of a single piece. In other sharks where the spiracle is minute the 

 cartilaginous support may be absent {Lainna). In the rays (Torpedo, fig. 63; 

 Raja, fig. 69; Myliohatis) it is well developed, and serves as a framework for 

 the support of the spiracular valve. It follows a true prespiracular ligament. 



The liyoid or second visceral arch, which is simple in primitive sharks, is 

 subject to great modification when the Elasmobranchs in general are con- 

 sidered. This arch in its generalized condition (Heptanchus) is composed of 

 a dorsal segment which suspends a ventral segment. Under such conditions 

 the hyoid does not function, or functions but slightly, in suspending the first 

 or mandibular arch. The proximal end of the dorsal segment consequently is 

 attached loosely by ligaments and hence indents the auditory capsule but 

 slightly as in Heptanchus cinereus. In other types it may indent the capsule 

 by only a part of its surface (Hexanchus, Chlamydoselachus) . When this oc- 

 curs the distal end of this segment suspends the ventral segment of the hyoid 

 and the latter is bound by ligaments to the mandible. 



In general, an attachment of the second arch to the mandibular may be 

 made by a ligament at the joint and elsewhere. In Chlamydoselachus the form 

 of attachment is very simple. While the lower segment depends slightly upon 

 the mandible, the latter begins also to depend upon the upper segment for 

 support. In other words, the dorsal segment is on its way to become a hyoman- 

 dibula or suspensorium. Where it is a true hyomandibula, as in most sharks, 

 this upper segment of the hyoid arch is of service primarily to the first arch. As 

 a suspensorium it becomes stronger and its articulation with the cranium l)e- 

 comes deeper; furthermore, its ligamentous attachment to the mandibular 

 arch may be most complex (Heterodontus francisci, fig. 67; see Daniel, 1915) . 



When the upper segment of the hyoid has assumed the secondary function 

 of suspending the mandibular arch, that is, where it becomes hyomandibular, 

 it may still continue to suspend the lower part of its owai arch also. This is 

 indeed characteristic of the sharks. But the lower segment may lose connec- 

 tion more and more with the upper (hyomandibular) segment. Instead of 

 being suspended from the distal end of the hyomandibula, it may be joined 



