A GILL OF WATER. 53 



lower jaw, which is usually composed of two pieces, 

 a dental and articular portion, 34 & 35. 



Besides the somewhat complicated apparatus of 

 the branchial arches, the hyoid bone bears on each 

 side of it certain osseous rays, 43, which support 

 the branchial membrane ; and a peculiar kind of 

 lid or clapper, commonly called the gill cover, and 

 composed of three bony pieces (the opercle 28, 

 the sub-opercle 32, and the inter-opercle 33) com- 

 bine with that membrane to close the great open- 

 ing of the gills. This covering articulates with 

 the tympanal, 27, and plays upon the pre-opercle, 

 30. In the Chondropterygian groups, however, 

 the general structure of these parts is much more 

 simple. 



The three opercular pieces just mentioned, do 

 not of themselves effect the closure of those great 

 clefts observable on each side of a fish, between 

 the head and shoulder, and within which are the 

 respiratory organs or branchiae. This closure is 

 completed by the branchiostegous membrane, which 

 adheres to the hyoid bone. This bone is placed as 

 in other vertebrated animals, but is always sus- 

 pended to the temporal bones. It is composed of 

 two branches, each consisting of five pieces, viz. the 

 styloid, by which it is suspended to the temporal; 

 two large lateral pieces, 37* & 38, placed one be- 

 hind the other, and forming the principal portion 

 of the branch (the posterior, 38, being that which 



* No. 37 is invisible in our woodcut. The same may be noted re- 

 garding one or two other bones, enumerated in a preceding note, the 

 names of which are inclosed in parenthesis. 

 E 



