498 



PISCES FISHES. 



present spines and indentations, which, being visible externally, are 

 of much importance to the icthyologist, as they afford a good 

 character of distinction between allied genera. The second piece 

 (28), which from its size is called par excellence the Operculum, 

 together with the Sub-operculum (32) and the Inter-operculum 

 (33), form a flap which covers the gill-opening like a great valve, 

 opening and shutting continually to give exit to the water used in 

 respiration. 



(534.) Lower Jaw. The lower jaw of fishes consists of two 

 lateral halves united by a symphysis in the mesian line, each 

 branch being articulated with the jugal bone of its corresponding 

 side. Each division is separable by maceration into four or even 

 five pieces : viz. the Dental (34), which supports the teeth; the 

 Articular ($5), bearing the articulating facet ; the Angular (36), 

 forming the angle of the jaw ; and a fourth, placed upon the inner 

 surface of the articular, called the Opercular, because it corre- 

 sponds with a bone met with in the lower jaw of reptiles, to which 

 the same name has been applied. The fifth, when present, is very 

 small and unimportant. 



aides and Bran- 

 chiostegous Rays. 

 The Os Hy- 

 oides of a fish 

 is situated as in 

 other vertebrate 

 animals ; it is 

 composed of two 

 branches, each 

 made up of seve- 

 ral pieces (37, 

 38, 39, 40), and 

 is always suspend- 

 ed from the tem- 

 poral by means of 

 two small ossi- 

 cles (59), which, 

 as they represent 

 the styloid pro- 

 cess of man, are 

 called the Styloid bones. 



