SKELETON. 55 



articulary cavity at its upper anterior angle for its jvmction 

 with the hyomanclibular. The oblong lamella below the 

 operculum is the subopcrculum (32), and the one in front of 

 this latter, below the horizontal limb of the prseoperculum, 

 is the interojjerculum (33), which is connected by ligament 

 with the angular piece of the lower jaw, and is also attached 

 to the outer face of the hyoid, so that the gill-covers cannot 

 open or shut without the hyoid apparatus executmg a corre- 

 sponding movement. 



The chain of flat bones which, after the removal of the 

 temporal muscles, appear arranged within the inner concavity 

 of the praeoperculum (Fig. 24), are comprised with the latter 

 under the common name of mandibular y suspcnsorium. They 

 connect the mandible with the cranium. The uppermost, the 

 epitympanic or hyomandihular (23), is articulated by a double 

 articulary head with the mastoid and posterior frontal. An- 

 other articulary head is destined for the opercular joint. The 

 mesotympanic or symplectic (31) appears as a styliform prolonga- 

 tion of the lower part of the hyomandihular ; is entirely 

 cartilaginous in the young, but nearly entirely ossified in the 

 adult. The position of this bone is noteworthy, because, 

 directly inwards of its cartilaginous junction with the hyo- 

 mandihular, there is situated the uppermost piece of the 

 hyoid arch, the stylo-hyal. The next bone of the series is the 

 pretympanic or metapterygoid (27), a flat bone forming a bridge 

 towards the pterygoid, and not rarely absent in the teleosteous 

 sub-class. Finally, the large triangular hypotympanic or 

 quadrate (26) has a large condyle for the mandibulary 

 joint. 



The palatine arch (Fig. 26) connects the suspensorium with 

 the anterior extremity of the skull, and is formed by three 

 bones : the entopterygoid (25), an oblong and thin bone attached 

 to the inner border of the palatine and pterygoid, and increas- 

 ing the surface of the bony roof of the mouth towards the 



