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 iakes 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 {Tetraodo7i), have the jaws divided in the 
centre by a perpendicular suture, and the skin covered with 
small, slightly prominent spines. The Triodons ( Triodoii), 
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 Molae ( Orthagonsciis), 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 {Batistes) 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), 
^ ave 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 
