128 



SUB-CLASS ELASMOCBRAXCH1I. 



/divided into two pieces ; of these the -dorsal-farms the skeleton 

 of the upper jaw, and is called the palato-jpiadrate bar, while the 

 ventral constitutes the lower jaw and is called the cartilage of 

 Meckel. The dorsal posterior end of Meckel's cartilage articulates 

 with the hind and ventral end (quadrate portion) of the palato-. 

 quadrate bar, while the ventral anterior end meets its fellow at 

 the symphysis of the lower jaw. Anteriorly the palato-quad- 

 rate bar joins its fellow beneath the ethmoid region but its rela- 

 tion to the skull varies in the most remarkable manner throughout 



FlG. 73. Skull of Hexanchus with mandibular and hyoid arch (after Gegen- 

 baur). P-Q palato-quadrate ; Hm hyomandibular ; Md Meckel's cartilage ; 

 C basihyal, L, L' labial cartilages ; p palatine process of palato-quadrate ; 

 M lateral process of ethmoid ; JV nasal sac ; Po postorbital process ; Gp fora- 

 men for glossopharyngeal nerve. 



the group. In the Notidanidae (Fig. 73) it articulates with 

 the skull at two points ; anteriorly it has a process, the palatine 

 process, which is connected with the skull between the exit of the 

 optic and trigeminal nerves, while posteriorly it articulates with 

 the postorbital process of the auditory cartilage, i.e. anterior to 

 the spiracle. In these skulls the hyoid arch, which bears 

 branchial rays, is attached to the auditory region and is 

 segmented into two pieces on each side and a median piece, 

 the hypohyal or copula. 



In other Selachians the palatine process is present, though the 



