NATURAL HISTORY. 
147 
ROOM IV.] 
cording to the structure of their outer covering. The true File Fish 
(Balistes ) 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 family of Trunk Fish (Ostracionim:) has the same elongated 
form as the Three-spined File Fish, and the body covered with an in¬ 
flexible case, formed of regular bony compartments. It has a very large 
liver, which yields a considerable 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 maxillary or inter¬ 
maxillary bones, their teeth being inserted on the palate and vomer. 
Their skeleton is essentially cartilaginous, the calcareous matter being 
deposited in the cartilage*! n a granular form, and not in threads or fila¬ 
ments ; their skull is composed of a single piece, without any suture. 
The Sturgeons (Acipenserid^e) have the gill-flap open like the other 
fishes. Their body is protected by bony plates implanted in the skin, 
and arranged in longitudinal rows ; their mouth is small, destitute of 
teeth, and placed at the base of an elongated muzzle. They ascend 
large rivers to spawn, and furnish one of the most profitable fisheries. 
Their flesh is excellent; their roe, dried and salted, forms caviar, and 
their swimming bladder, merely washed and dried, is the common isin¬ 
glass of commerce. The Spatularise ( Polyodon ) have a free gill-flap, 
like the Sturgeons, but their beak is long and spatula-shaped, and the 
mouth large and armed with teeth. 
The Chimerse (CfflMiERADiE) have great affinity to the Sharks both 
in external form and the position of their fins, but their gill cavity opens 
externally by a single hole on each side, and is covered by the rudi¬ 
ments of a gill-flap. Between their eyes they have a fleshy process ending 
in a group of small spines. They lay very large eggs, with a coriaceous 
shell, of an ovate-lanceolate shape with a fringed edge. 
All the other cartilaginous fishes have their gills adherent to the outer 
side of the gill cavity, allowing the water to escape through a series of 
holes between each gill. 
In most of these, as the Sharks and Rays, the gills are laminar. 
The fishes are furnished with large pectoral and ventral fins, and the 
mouth, which is usually placed under the end of the muzzle, is armed 
with teeth. 
The Sharks ( Squ alidje) are distinguished by their elongated form, 
and large fleshy tail, and by the gill aperture being placed on the side of 
the neck. Many of them are viviparous ; others produce eggs inclosed 
in a hard, horny shell. They have been divided into several genera ac¬ 
cording to the form of the nostrils, the position of the fins, and the ab¬ 
sence or presence of the aperture behind the eyes. Some, as the Ces¬ 
sations and Spine Sharks, have a large bony spine in the front of the 
dorsal fin. 
