ROOM XI.] 
NATURAL HISTORY. 
57 
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 ( Diodontes ) have both jaws undivided, and 
the skin armed with large spines. The skin of some 
species of this genus is used as a kind of helmet by the 
Chinese. The Tetraodons ( Tetraodontes), have the jaws di¬ 
vided in the centre by a perpendicular suture, and the skin 
covered with small, slightly prominent spines. The Trio- 
dons ( Triodonies) have the skin of the Tetraodons, but the 
upper jaw alone is divided, so that they appear to have 
three teeth. 
The Moon Fish, or Molee ( Orthagorisci ), 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 flsh. 
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 ), 
have the skin covered with small, hairy scales, and the 
first dorsal fin has only one spine. The Three-spined File 
Fish ( Triacantlius ) has a silvery skin covered with small 
scales, and a ventral fin, consisting of a single sjfine, on 
each side. 
The Trunk Fish (Ostracion) has the same elongated 
form as the Three-spined File Fish, and the body covered 
with an inflexible case, formed of regular bony compart¬ 
ments. It has a very large liver, which yields a con¬ 
siderable quantity of oil ; but very little flesh. The body 
is often armed with spines, and according to its form, 
and the position of the latter, the species have been 
distinguished. 
The cartilaginous fishes are entirely without any maxil¬ 
lary or intermaxillary bones, their teeth being inserted on 
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