296 FISHES. 



that, when these bones are wanting they are transformed into oper- 

 cular pieces. 



The under jaAvs of fishes do not vary less in their forms, and are no 

 less constant in their composition than the crania, and upper jaws ; at 

 one time perfectly transver, at another parabolic, or rounded in front, 

 now forming an angle more or less acute, their symphisis is sometimes 

 prolonged into a thin sharp point, as in the gar fish; and again as in 

 the hemiramphe this point is prolonged, far beyond the upper jaw. 

 The inverse, however, more commonly occurs. 



In resuming the account of the bones of the face, we find eighteen 

 or nineteen pairs constant, vid. one at the nasals, two at the upper 

 jaw, seven in the palato-temporal apparatus, four in the opercular 

 apparatus, and four or even five in the lower jaw, to which must be 

 added the suborbitars, and the supertemporals, which in the perch 

 form eight other pairs; and adding to these the bones of the cranium, 

 we shall find for the whole head, a total of nearly sixty bones or 

 thereabouts ; but the subdivisions to which the superior maxillary is 

 liable sometimes materially augment this number. 



The Os Hyoides and Branchiostegal Rays, (a) 

 The three opercular pieces alone do not close up the great slit, 

 one each side between the head and shoulder of the fish where 

 the branchies are lodged ; the closure is completed by the branchi- 

 ostegal membrane which adheres to the os hyoides. This bone 

 (pi. III. fig. vi. and vii) situated as in the other classes of verte- 

 brated animals, but always suspended to the temporal, is com- 

 posed of two branches consisting each of five pieces, vid. the 

 small styloid (No. 29) which suspends it to the temporal* two 

 large lateral pieces (Nos. 37 and 38) placed one behind the other, 

 and forming the principal body of the branch (it is the posterion of 

 the two (No. 38) f that is attached to the interoperculum) ; and lastly 

 two small ones (Nos. 39 and 40) placed one above the other at the 

 anterior extremity of the branch, and serving to join it with its 

 fellow.;]; The lingual bone (No. 41) is placed in front of this junc- 

 tion, as in birds and reptiles, and behind in the angle fo:med by the 

 meeting of the two branches, and beneath the branchies, there is an 

 azygous piece usually vertical (No. 42) which represents the tail of 

 the hyoides, so well known in birds and lizards. |] It is this piece, 

 which, by joining itself to the symphisis of the numerals forms 

 the isthmus which separates below the two openings of the ear. 

 The os hyoides of fishes forms the total number of twelve bones. 



* The style-hyal of M. Geoffroy. 



t M. Geoffroy who thinks these two principal pieces are derived from the Sternum, 

 calls the anterior hyo-sternal, and the posterior hi/posternal. 



X The superior of these two small pieces is called by M. Geoffroy apo-hyal, and 

 the inferior the cerato-hyal, because he considers them as answering to the two first 

 pieces of the anterior horn of the os hyoides in Mammalia. 



|| It is this azygous vertical piece, that M. Geoffroy regards as analogous to the 

 anterior azygous apophysis of the sternum in birds, and on which account he calls 

 it episternal, but the episternal of birds is always situated behind the fmirchette which 

 is their small clavicle. 



(a) A side view of the os hyoides, is represented pi. II. fig. vi., and plate III. 

 fig. i. ; and a view from above, with the bones of the branchiae, pi. III., fig. vi. 



