PLECTUGNATIII; FAMILY G*M>UrMJMJC,3 ; DiOUOiN, SUN-FISH. 45 



F. PIIYSOSTOMI, with soft fin-rajs, except the first in the 

 dorsal, anal, and pectoral fins, which are occasionally spinous ; 

 and an air-bladder which communicates with the pharynx by an 

 open duct. 



A. PLECTOGNATHT. 



$ 



604. This group, which in many particulars seems to ap- 

 proach the Selachian and Ganoid Fishes, includes only two 

 families, distinguished by the mode in which the jaws are armed. 

 In the first of these, the GYMNODONTES, or Fishes with naked 

 teeth, the jaws are covered with plates of a substance resembling 

 ivory. These are reproduced as fast as they are destroyed by 

 wear ; and they constitute very efficient instruments for grind- 

 ing down the food on which these animals live, which consists 

 of Crustacea and sea-weeds. To this family belong the Globe- 

 fishes; which are so named from their power of distending them- 

 selves into a spherical form, by inflating with air a large sac 

 contained in the abdomen. When thus distended, they float 

 along the water with the back downwards, swimming onwards 

 by means of their pectoral fins ; and they are covered with a 

 series of large spines, which are raised up when the body is thus 

 inflated, so as to form a very efficient means of defence. From 

 this last circumstance, these Fishes have been sometimes termed 

 Pjarcupine-fish. There are three genera in which this curious 

 power exists ; the Diodon (two-toothed), in which there is no 

 furrow or division in the jaws, so that each seems like a single 

 tooth ; the Triodon (three-toothed), in which there is a division 

 in the centre of the upper jaw ; so as to divide it, as it were, into 

 two teeth; and the Tetraodon (four-toothed), in which there 

 is a division of this kind in each jaw. Besides these, this family 

 contains the Orthagoriscus, or Sun-fish (so named from its 

 rounded form), which looks like the anterior half of a fish cut in 

 two in the middle. It has the power of floating with its head 

 and eyes above water ; but not of distending itself with air ; in 

 this state it moves along sideways, very slowly, however ; anil 

 appears like a dead or dying fish. The Sun-fish (or Moon-fish, 

 as it is sometimes called) attains a considerable size ; of the short 



