VISCERAL ARCHES. 



129 



connection is further forwards with the ethmoid region (ethmo- 

 palatine ligament), but the posterior articulation, viz. with 

 the postorbital process is not present. On the other hand the 

 posterior end of the bar is attached to the dorsal element of 

 the hyoid arch by ligaments. 



In such Selachii the condition of the hyoid is very similar to 

 that in the Notidanidae (already described), excepting that the 

 dorsal piece is much stouter and assists to a marked extent by 

 means of ligamentous bands, attaching it to both the quadrate 

 end of the palato- quadrate bar and to Meckel's cartilage, in 

 supporting the mandibular arch. This dorsal piece is for this 

 reason called the hyomandibular, the other piece being called 

 the ceratohyal. In Raji the palatoquadrate bar is without 



Fig. 74. — Diagram showing tlie principal modifications in the arrangement of the hyoid arch 

 in Plasiostomes (after Gegenbaur). A in Notidanus ; B in pentanchal Selacliians ; C in 

 Torpedo ; D in Raja. Hm hyomandibular ; p process of Um ; hy lower part of hyoid 

 arch ; r branchial rays. 



the palatine process, and the mandibular arch is entirely sup- 

 ported by the hyomandibular. This structure is without gill- 

 rays and has developed a process for the support of the mandibu- 

 lar arch (Fig. 74, p). It has almost lost its relation to the 

 lower part of the hyoid arch, which is connected only to its 

 dorsal end, or is attached dorsalty direct to the auditory cartilage 

 (many Rajidae, etc.). The hyoid arch of such forms is only 

 equivalent to the ventral part of the hyoid of the Notidanidae 

 and Squali generally. It is a comparatively slender, jointed 

 structure, closely resembling the posterior branchial arches, 

 like which it bears branchial rays. There are frequently two 

 or three small cartilages, the prespiracular cartilages, in the 

 anterior wall of the spiracle. 



Z-II K 



