58 NATURAL HISTORY. [UPPER FLOOR. 
the palate and vomer. Their skeleton is essentially carti¬ 
laginous, the calcareous matter being deposited in the 
cartilage, in the form of grains ; their skull is composed of 
a single piece, without any suture. 
The Sturgeons ( Acipenseres ) 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 isinglass of commerce. 
The Spatulariae (Polyodontes ) 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 ( Chimceroe ) 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 (a Squalidce), 
the gills are laminar. The fish 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 ( Squall ) are distinguished by their elon¬ 
gated 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 absence or presence of the aperture behind the 
eyes. Some, as the Cestrations and Spine Sharks, have a 
large bony spine in the front of the dorsal fin. 
The Hammer-headed Shark (Zygcena) is peculiar for 
the head being flattened, truncated in front, and extended 
horizontally on the sides, so as to resemble a hammer. 
