54 NATURAL HISTORY. [UPPER FLOOR. 
and consequently immoveable. Their skeleton is soft, but 
fibrous. 
Cases 7 and 8. The first of these, the Gymnodontes, 
have the jaws shaped like the beak of a parrot, and com¬ 
posed of parallel laminae united together. They live on 
Crustacea, shells, and sea-weeds, and their flesh, which 
has usually a musky odour, is said to be deleterious at 
particular seasons. Several of these fishes have the faculty 
of dilating their stomach with air, giving the body the 
.appearance of a balloon. When this takes place they float 
along the surface of the water, in an inverted position. 
The Diodons ( Diodon ) have both jaws undivided, and 
the skin armed with large spines. The skin of some 
species of this genus is said to be used as a kind of helmet 
by the natives of the north-west coast of America. The 
Tetrodons ( Tetraodon ), have the jaws divided in the 
centre by a perpendicular suture, and the skin covered with 
small, slightly prominent spines. The Triodons ( Triodon), o 
have the skin of the Tetrodons, but the upper jaw alone 
is divided, so that they appear to have three teeth. 
The Moon Fish, or Molse ( Orihagoriscus), have the 
same kind of jaws as the Diodons, but the body is com¬ 
pressed and without spines, and not susceptible of being 
inflated, and the tail is so short and high, that they have the 
appearance of being merely the head of a larger fish. 
The File Fishes ( Balistidce ) have their jaws armed 
with a small number of distinct teeth; their skin is 
hard, and their head produced, ending in a small mouth. 
They are divided into several genera, according to 
the structure of their outer covering. The true File 
Fish [Ballstes) has a compressed body, covered with hard 
scales, and the first dorsal fin has spinous rays; its 
colours are brilliant, but its flesh is said to be unwholesome. 
It is chiefly found in the Torrid Zone, living upon sea¬ 
weeds. Others, as the Unicorn File Fish [Monacanthus), 
have the skin covered with small, hairy scales, and the 
first dorsal fin has only one spine. The Three-spined File 
Fish ( Triacanthus ) has a silvery skin covered with small 
scales, and a ventral fin, consisting of a single spine, on 
each side. 
The Trunk Fish ( Ostracion ) has the same elongated 
form as the Three-spined File Fish, and the body covered 
