26 NATURAL HISTORY. 



entirely of cartilage, and terminates backward in two oblong processes called the occipital con- 

 dyles, by which it joins on to the backbone or vertebral column. In this character there is a resem- 

 blance to the amphibia and to mammals ; but the cartilaginous skull is covered with a thin layer of 

 bone, which is deposited in cells like a minute tesselated pavement or honey-comb. There are large open 

 spaces covered with membrane in the upper part of the skull. The upper ja\v is not subdivided into 

 separate bones any more than the skull ; and the lower jaw also, unlike that of all other fishes, and 

 like that of mammals, consists of a single piece of bone on each side. As there is never any cover to 

 the gills, the opercular bones are also wanting. There are two pairs of lateral fins, which cor- 

 respond to fore arid hind limbs. The teeth vary in character. In the Sharks they are commonly 

 sharp and pointed, and adapted for seizing and biting, but in the Rays they are more frequently 

 blunt, and adapted for crushing. They are replaced when worn. These fishes never have an air- 

 bladder j the intestine is always short, and terminates in a spiral valve. The part of the heart 

 termed the aortic bulb is remarkable for containing three sets of valves for controlling the circulation. 

 A great many Sharks and Rays have a tube leading from the mouth to the upper side of the head, 

 which is called a, spiracle. In the Sharks the gills open by five or seven vertical slits on the sides of the 

 head. In the Rays there are always five pairs of gill-openings, which are placed on the under side 

 of the body. The Saw-fishes connect these two groups. The brain is chiefly remai-kable for the 

 large size of the cerebellum, and the great development of the olfactory lobes. Usually there are 

 two ovaries ; but in certain Sharks there is but one. The eggs are large and few, but are laid only 

 by Dog-fishes and Rays. The great majority of Sharks bring forth their young alive, and the young 

 are sometimes attached to the body of the parent. The claspers of the males are often large organs. 



SUB-ORDER I. SELACHOIDEI, OR SHARKS. 



The Sharks all have a more or less cylindrical body, which tapers to the snout and contracts 

 gradually into a tail. The group has been divided into nine families, though the characters by 

 which they are distinguished depend chiefly upon external characteristics. The families are named 

 from typical genera Carchariidse, Lamnidse, Rhinodontidse, Notidanidse, Scylliidse, Cestraciontidse, 

 Spinacidse, Rhinidse, and Pristiophoridse. 



FAMILY I. THE CARCHARIIDJE. 



These fishes form a large family distinguished by having an anal fin and two doi-sal fins 

 and a nictitating membrane, or third eyelid, like that of birds. The family includes eleven 

 genera. The first dorsal fin is always placed opposite the space between the pectoral and ventral 

 tins, and never carries a spine on its anterior mai'gin. Dr. Giinther has divided the family into 

 several sections, some of which may be regarded as themselves forming families. In the genus 

 Carcharias, in which there is always a pit at the root of the tail, and never any spiracles, the 

 teeth, which are more or less triangular, have a single sharp cusp ; they extend round a crescent- 

 shaped mouth. In one section of the genus the teeth are more or less denticulated, but in the other 

 section the teeth show no trace of serrations on their cutting margins. In Carcharias, Dr. Giinther 

 recognises thirty-five species, which have been classed by the characters of their teeth into as many as 

 five sub-genera. The majority of these fishes are known from tropical seas. Their snouts are some- 

 times greatly elongated, but frequently blunt. The number of teeth varies in the different species, and 

 is not always quite the same in both jaws, but usually ranges between twenty-five and forty-nine ; 

 the difference is rarely more than two in the upper and lower jaw, and frequently there is only a 

 difference of one. Occasionally the excess is in the upper jaw, but more frequently in the lower jaw. 

 The Blue Shark of English shores belongs to the section with serrated teeth. 



THE BLUE SHARK.* 



Towards Midsummer the fishermen on the Cornish coasts often find their nets and lines attacked 

 by the Blue Shark. It follows the Pilchards and Herrings, and frequently bites out the part of the net 

 in which the fish are entangled. When the bait on a line has been swallowed, and the fish has failed to 

 bite the line through, it often rolls the rope round its body until it reaches the surface, coming up in 



* Carcharias glaucvj. 



