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CHAPTER II. 

 THE PAL^EICHTHYES, OR FISHES OF ANCIENT TYPES. 



DIPNOI, OR MUD -FISHES Why this Order is Interesting THE AFRICAN MUD-FISH THE SOUTH AMERICAN MUD- 

 FISH _THE GENUS CERATODUS GANOIDEI, OR FISHES AVITH BONY SCALES THE AMIID* Amia calva 

 THE BONY PIKE OF THE NILE THE AMERICAN BONY PIKE Its Remarkable Characters HOLOCEPHALA The 

 Chimcera monstivsa The Genus Callorhynchus PLAGIOSTOMATA The Sharks and Rays SELACHOIDEI THE 

 BLUE SHARK Its Habits Muscular Vitality Economic Uses Other Genera The Common British Tope THE 

 HAMMER-HEADED SHARK THE SMOOTH HOUND The Porbeagle, or the Beaumaris Shark THE THRESHER, OR Fox 

 SHARK THE BASKING SHARK Enormous Proportions The "Sea Serpent" Habits Fisheries Characters THE 

 SIX-GILLED SHARK THK DOG FISHES The Nurse Hound and the Rough Hound Their Eg,*s and Egg-purses 

 " Sea-dog Soup "The Black-mouthed Dog-fish- Cestracion THE PICKED DOG-FISH Why so Named Characters- 

 Various Forms THE SPINOUS SHARK THE MONK-FISH The Indian Shark Oil Industry BATOIDEI, THE RAYS 

 Distinctive Features THE PRISTID.E THE RHINOBATID.E THE TORPEDINID.E THE GENUS TORPEDO Strength 

 of the Shock The Electric Organs Characters Other Forms -THE RAYS Characters -The True Skate Fishery 

 THE LONG-NOSED SKATE THE BORDERED RAY THE SHAGREEN RAY THE HOMELYN RAY THE THORNBACK 

 THE STING RAYS Growth of its " Spine "Various Species THE EAGLE RAY THE Ox RAY, OR SEA DEVIL 

 CHONDROSTEI THE STURGEONS Characters -Caviare Fishery Other Economic Uses The Common Sturgeon- 

 Article of Diet- The Acipenser /zwso-The Sterlet The Paddle-fish. 



DIVISION I. PAL^EICHTHYES. 



ORDER L DIPNOI. MUD-FISHES. 



THIS small group of fishes has more than ordinary interest, from the circumstance that it com- 

 prises surviving representatives of a large fish fauna, now entirely extinct, which abounded in 

 the early periods of the earth's history. And yet, instead of presenting, as might have been 

 expected, characters of immature or imperfectly-developed forms, these fishes are the only ones 

 which make an appreciable approximation in structure to the Amphibia. 



FAMILY SIRENOIDEI. 



There are only three genera known Protopterus, limited to Africa, Lepidosiren to South 

 America, and Ceratodus to Australia. 



GENUS PROTOPTERUS. THE AFRICAN MUD-FISH. 



But one species is known, the Protopterus annectans, which has an Eel-like form, grows to a 

 length of three feet, and is found in the Nile, the Zambesi, and the Gambia. It has been 

 brought alive to Great Britain enclosed in balls of hardened clay, in which the fish hibernate 

 and remain torpid during many months of the year, with a small hole in the clay at each end to 

 admit the air. They -are abundant in the rice-fields, where they are dug out of the mud by the 

 natives, who regard them as a delicacy. 



The examples originally described by Sir R. Owen were twelve or thirteen inches long, and 

 measured about four inches and a half round the body. The head was two inches long, and the 

 distance from the pectoral to the ventral fin five inches and a half. The muzzle is blunt, and the 

 head gradually enlarges towards the gill-opening, which is just in front of the base of the pectoral 

 fin or fore-limb. A line of mucous pores surrounds each eye, and from this the lateral line com- 

 mences, and is prolonged down the body to the end of the tail, making a slight downward curve 

 towards the ventral fins. There is a membranous dorsal fin. The body is sheathed in scales of 

 the cycloid pattern, which are arranged in about sixteen longitudinal series on each side of the 

 body. Each scale is marked by a number of canals, which radiate from a centre near the posterior 

 edge, and are connected by cross canals. The bones are green, like those of the common Garfish. 

 The vertebral column retains the primitive condition seen in the early stage of development of all 

 animals in which the continuous cylindrical, somewhat gelatinous rod, which is termed the notochord 

 persists, in the position which usually becomes occupied by the bodies of the vertebrae, a change which 

 is brought about by the deposition within the notochord of the salts of lime which form bony matter. 

 Here the notochord has merely an external sheath of ligament, except towards the tail, where it 

 becomes somewhat cartilaginous. The neural arches, however, which cover the spinal cord, are con- 

 verted into bone, and are prolonged into neural spines, each of which articulates with a bone above, 

 which bones form the base of the dorsal fin. There are thirty-six pairs of simple ribs, which are all of 



