58 SUPPLEMENT ON 



(No. 2(5) ; above this, and behind the pterygoidean, is another 

 broad, flat, and thin bone, named tympanal (No. 27), and 

 above this the large temporal bone (No. 23), already noticed. 

 This bone affords backwaids a tubercle to articulate with 

 the principal piece of the operculum (No. 28), and below 

 furnishes an attachment for a bony stylus (No. 29), which 

 bears the branch of the hyoid bone, and represents the styloid 

 of mammiferse. Behind these three pieces is placed length- 

 ways the bone (No. 30), which constitutes the fixed border 

 for the movements of the operculum, and which is therefore 

 the preoperculum. But there is besides a long narrow bone 

 (No. 31), between the intermediate flat bone and the pre- 

 operculum, sliding partly behind that which articulates with 

 the lower jaw, and forming an angle with the styloid : this is 

 named the symplectic. 



The opercular bones consist of the preoperculum (No. 30), 

 usually a right angled l bone, encompassing the posterior bor- 

 der and angle of the main blade of the palato-temporal, and 

 belongs to it more than to the opercular apparatus. The 

 serrse and spines, which often appear on the border and angle, 

 being visible, are of great use in determining the characteristic 

 distinctions of fishes. The operculum, properly so called, 

 (No. 28), is placed behind the ascending border of the former 

 bone, and moves upon it like a shutter within its case. The 

 suboperculum (No. 32), is placed beneath the posterior and 

 inferior border of the operculum ; and before this, under the 

 preoperculum, and behind the lower jaw, is the interoper- 

 culum (No. 33). This bone furnishes an attachment to the 

 branch of the hyoid bone, at the point where it is itself 

 joined to the styloid, which suspends it on the temporal bone ; 

 hence the opercular shutters cannot open or close without a 

 corresponding movement of the hyoidean branches. 



1 That is a bone bent at a right angle, so as to resemble the instrument 

 used by carpenters, and termed a square. 



