PLECTOGNATHI 



725 



among fishes. According to Brown Goode, " the propelling 

 force is exerted by the dorsal and anal fins, which have a half 

 rotary, sculling motion, resembling that of a screw propeller ; the 

 caudal fin acts as a rudder, save when it is needed for imusually 

 rapid swimming, when it is used as in other fishes ; the chief 

 function of the broad pectorals seems to be that of forming a 

 current of water through the gills, thus aiding respiration, which 

 would otherwise be difficult on account of the narrowness and in- 

 flexibility of the branchial apertures. When taken from the 

 water, one of these fishes will live for two or three hours, all the 

 time solemnly fanning its gills, and when restored to its native 



Fig. 438. — Ostracioii qtiadricornis. x J. 



element seems none the worse for its experience, except that, on 

 account of the air absorbed, it cannot at once sink to the bottom." 

 " ISTo group of tropical fishes," says the same author, " is so 

 thoroughly worked out in the writings of the fathers of natural 

 history as this one. Over 200 years ago every species of trunk- 

 fish now taken from the Atlantic was known to and described by 

 the naturalists, and it is a well-deserved tribute to their dis- 

 crimination as zoologists to say that none of the many efforts 

 which have since been made to subdivide their species have been 

 at all successful." 



Division II.— GYMNODONTES. 



Supraclavicle oblique, sometimes nearly horizontal; lower three 

 pectoral pterygials enlarged and immovably united to the coraco- 

 scapular cartilage ; upper pterygial small, suturally united to the 

 scapula. Anterior vertebrae with bifid divergent neural spines. 

 Basis cranii simple ; suture between dentary and articular evident. 



Pelvis absent. 



The spinous dorsal and the ventral fins are constantly absent, 

 the praemaxillaries are united to the maxillaries, and the teeth 



