THE ENDOSKELETON 



153 



The first, or mandibular pair, which is the most modified 

 of all, becomes folded about the mouth in such a way as to 

 make a serviceable pair of jaws, both upper and lower, with 



IU 



B 



FIG. 40. Skull and bran- 

 chial skeleton of Squalus acan- 

 thias, the dog-fish, after draw- 

 ings by students. 



(A) Lateral view. [C. E. Hep- 

 burn.] (B) Ventral view. [J. L. 

 Whitney.] 



PPQ, palato-pterygo-quadrate; 

 Mk. Meckel's cartilage; K. I. the 

 two sets of labial cartilages; HM, 

 hyomandibular ; CH, cerato-hyal; 

 Sp, spiracular cartilage; BB, basi- 

 branchial; HB, hypo-branchial ; CB. 

 cerato-branchial ; EB, epi-branchial; 

 PB, pharyngo-branchial. The sub- 

 script numerals designate the sepa- 

 rate gill arches, the Roman numer- 

 als the visceral arches. 



YII 



each of which several rows of pointed placoid scales are as- 

 sociated to serve as teeth. In short, one hardly knows whether 

 to describe these parts as gill-arches covered with placoid 



