576 MAND1BULAR AND HYOID BARS. 



mandibular arch that it is never provided with an unpaired basal 

 element. 



The simplest forms of metamorphosis are those undergone 

 by Elasmobranchii, of which the Dog-fish (Scyllium) and Skate 

 (Raja) have been studied (Parker, No. 456). In some of these 

 forms, e.g. the Skate, part of the mandibular bar is still related to 

 the hyo-mandibular cleft (the spiracle). 



Elasmobranchii. In Scyllium the hyoid and mandibular 

 arches are at first very similar to those which follow. Soon 

 however each of them sends an anteriorly directed dorsal process 

 (fig. 329). The regions which may be distinguished owing to the 

 growth of these processes have received names from ossifications 

 in them which are found in other types. The anterior process of 

 the mandibular arch is known as the pterygo-quadrate bar 

 (Pl.Pt) ; the dorsal end of the primitive bar from which it starts 

 (M.Pt) is known as the metapterygoid process; while the 

 ventral end of the bar forms the Meckelian cartilage. The 

 upper end of the hyoid arch is known as the hyomandi- 

 bular. 



In a somewhat later stage changes take place which cause 

 these parts practically to assume the adult form (fig. 327). The 

 mandibular arch becomes segmented at its bend into (i) a 

 pterygo-quadrate bar {Pl.Pt) which grows forwards in front of 

 the mouth, and forms an upper jaw, and (2) a Meckelian cartilage 

 (Mck) which is placed behind the mouth, and forms a lower jaw. 

 The two jaws are articulated together, and the cartilages of the 

 two sides composing them meet each other distally. 



At the articulation of the Meckelian cartilage with the quad- 

 rate part of the pterygo-quadrate is situated a ligament (M.Pt), 

 which takes the place of the metapterygoid process of the 

 previous stage, and passes up on the anterior side of the spiracle, 

 to be attached to the cranium in the front part of the auditory 

 region. This ligament, which is supplemented by a second 

 ligament, the ethmopalatine ligament, passing from the 

 pterygo-quadrate bar to the antorbital region of the skull, is not 

 the most important support of the jaw. The main support is, on 

 the contrary, given by the hyoid arch ; the hyomandibular 

 segment of which (H.M) as well as the adjoining segment (cera- 

 tohyoid C.Hy) are firmly attached by ligament to the mandibular 



