'222 PISCES. 



ORDER VI. 



PLECTOGNATHl*. 



WE have now passed from the preceding five orders of bony or fibrous 

 fishes, with free and complete jaws, to the sixth, which may be approximated 

 to the Chondropterygii, with which it is allied by the imperfection of the 

 jaws, and the tardy induration of the skeleton ; this skeleton, however, is 

 fibrous, and its whole structure is that of ordinary fishes. The most distin- 

 guishing character of the order consists in the maxillary bone being soldered 

 to the side of the intermaxillary, which alone constitutes the jaw, and in the 

 mode in which the palatine arch is connected with the cranium, which, being 

 by a suture, consequently renders it immoveable. Besides this, the opercula 

 and rays are concealed under a thick skin, through which only a small 

 branchial fissure is visible. Of ribs, nothing is to be found but very small 

 vestiges. There are no true ventrals. 



This order comprises two very natural families, characterised by their mode 

 of dentition. 



FAMILY I. 



GYMNODONTESt. 



THE Gymnodontcs have jaws, which, instead of teeth, are furnished with 

 an ivory substance, internally divided into laminae, whose en- 

 semble resembles the beak of a parrot, and which in fact consists 

 of true teeth united, that succeed each other as fast as they are 

 destroyed by triturition. The opercula are small, and there are 

 five rays on each side, all of which are but imperfectly seen. 

 They live on Crustacea and fucus ; their flesh is mucous, and 

 that of several species is considered poisonous, at least in certain 

 seasons. 



Two of the genera, Tetraodon and Diodon, have the faculty of swelling 

 themselves up like a balloon, by filling their stomach (or rather a sort of very 

 thin and extensible crop, which occupies the whole length of the abdomen, and 

 adheres closely to the peritoneum), a circumstance which has occasioned it to be 

 considered at one time as the peritoneum itself, anil at another as a species of 

 epiploon with air. When thus inflated, they roll over, and float on the 

 surface, with the abdomen upwards, unable to direct their course ; but they 

 are extremely well defended while in this position by the erection of the 

 spines with which their skin is everywhere furnished. 



Cheeks united by suture. ) Naked teeth. 



