50 
NATURAL HISTORY. [[UPPER FLOOR. 
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 
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 
the palate and vomer. Their skeleton is essentially carti¬ 
laginous, the calcareous matter being deposited in the 
cartilage, in a granular form, and not in threads or fila¬ 
ments ; their skull is composed of a single piece, without 
any suture. 
The Sturgeons ( Acipenser ) 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 
caviare, and their swimming bladder, merely washed and 
dried, is the common isinglass of commerce. 
The Spatularise {Volyodon') 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 Chimeree ( Chmczra ) 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 rudiments 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. 
All the other cartilaginous fishes have their gills ad¬ 
herent 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 ( Squalidce ), 
