634 PISCES. 



palate bone : one, situated externally, becomes, in Reptiles, a very im- 

 portant element ; it is called the transverse bone (24) ; the second (25) 

 is named the internal pteryy old. 



(1717.) The other pieces belonging to this part of the skeleton are 

 not a little interesting on account of their remarkable arrangement; 

 and perhaps the anatomical student will be somewhat startled at the 

 position which some of them occupy. In the first place, the squamous 

 portions of the temporal, instead of entering into the formation of the 

 cranium, are here slightly displaced, and, although still called the tem- 

 poral bones (23), are articulated by a hinge-joint with the posterior 

 frontal and mastoid bones, and thus form a moveable basis to which the 

 opcrcular apparatus is attached. 



(1718.) Connected with the temporal we have the broad and flat 

 piece, 27, which is the tympanic bone ; and to these the pieces forming 

 the opercula are appended. 



(1719.) Lastly, supporting the lower jaw we find the jugal bones ; 

 and connecting these with the rest of the temporal apparatus are two 

 small ossicles (31), which complete this portion of the skeleton. 



(1720.) The seven bones above enumerated are almost immoveably 

 connected with each other by the interposition of cartilage between 

 their edges a mode of articulation distinguished by the name of syn- 

 chondrosis', but the whole apparatus moves readily upon two hinges, 

 one formed by the articulation of the palate bone with the maxillary 

 and vomer, and the other by the joint which unites the temporal bone 

 to the posterior frontal. This movement, by opening the gill- covers, 

 enlarges the cavity of the mouth when the fish wishes to take in the 

 water necessary for respiration, or else, by acting in a contrary direction, 

 again expels it. 



(1721.) Opercular bones. The great flap, which in osseous fishes 

 closes the gill-openings externally, is composed of four pieces, to which 

 the following names have been given. The prceoperculum (30) is at- 

 tached to the posterior edge or angle of the palato- temporal apparatus 

 last described, and its borders often present spines and indentations, 

 which, being visible externally, are of much importance to the ichthyo- 

 logist, 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 suboperculum (32) and the interoperculum 

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

 ing and shutting continually to give exit to the water used in respiration. 



(1722.) Lower jaw. The lower jaw of Fishes consists of two lateral 

 halves united by a symphysis in the mesial 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 (35), bearing the articu- 

 lating facet ; the anyular (36), forming the angle of the jaw ; and a 



