22 NATURAL HISTORY. 



teeth. The tongxie is covered with papillae ; the nostrils are prolonged into short tubes. The scales 

 are of the cycloid pattern, and are sometimes covered with enamel ; they are large, and marked with 

 radiating lines, and are enveloped in a soft skin ; those which occur in the lateral line are slightly 

 elevated. The colour is dull, often dark greenish, with black spots and bands, and there is fre- 

 quently a round black spot on the tail. The animal is covered with thick mucus. Its movements in 

 the water are not very rapid. It feeds chiefly on fresh- water Crustacea, is sometimes eaten by the 

 Indians, and attains a length of about two feet. The vertebral column is chiefly remarkable for 

 having intercalary vertebrae introduced in the tail. The first appears after the sixth caudal, and 

 the last between the twenty-second and twenty-third caudal. These intercalary bones are entirely 

 devoid of processes, and occasionally one or more of them may be absent. The end of the vertebral 

 column is cartilaginous, and directed iipward. The air-bladder is a large membranous sac divided 

 anteriorly into two short horns ; its internal appearance is compared to that of the lung of a serpent 

 with cells in the anterior part which disappear towards the posterior end. It communicates with 

 the esophagus by a duct, and has a sort of glottis with an oblong opening. There are four gills; 

 each arch is formed by a double row of leaflets ; there are ten or twelve branchiostegal rays. The 

 stomach forms a blind sac, divei-ging from the intestine; there are no pancreatic appendages; the liver 

 has two lobes ; and there is a rudimentary spiral valve at the termination of the intestine. This fish 

 is limited to the fresh waters of the United States, and is especially met with in the great expanse 

 of low-lying country between the Alleghanies and the Rocky Mountains, in the Mississippi, Nor- 

 thern lakes, and Middle States. 



FAMILY II. POLYPTERID^E. THE BONY PIKE OF THE NILE.* 



This is the type of a family which at the present day includes only two genera, Polypterus 

 occurs throughout the tropical parts of Africa, especially in the Nile, Gambia, and Senegal rivers, and 

 other parts of the west coast. It is an elongated fish, with a short snout and somewhat cylindrical 

 body. It is defended with lozenge-shaped ganoid scales. The species Polyptertu bichir lives in the 

 mud at the bottom of the rivers, where the fish crawl or walk like Seals by means of their fins. They 

 swim with great rapidity, much in the manner of serpents. At the time of reproduction they are 

 chiefly at the surface of the water. This fish presents an extraordinary appearance, from the 

 way in which the dorsal fin is broken up into a succession of little finlets, which vary in number in 

 the several varieties from eight to eighteen. The vertebrae are bi-concave, as in ordinary fishes, 

 but the termination of the vertebral column is cartilaginous. The head is covered by enamel similar 



rOLYPTERVS. 



to that which defends the scales of the trunk. From the lateral expansion of the bones of the 

 head, this fish presents much the same sort of resemblance to a Chelonian that the head of the 

 Lepidosteus has to that of a Crocodile. The ventral fins are well developed, and the anal fin is 

 placed close to the lower margin of the caudal fin. The central portion of the fin in these fishes is 

 fleshy, and covered with scales, so that the rays appear as a fringe around it. This character is 

 met with in many of the fishes of the Old Red Sandstone, and Professor Huxley has proposed to 

 unite them together under the name of Fringe-finned Fishes, or Crossopterygidse. There are three 

 bones between the fin rays and the shoulder-girdle. The air-bladder is more simple than that of 

 Lepidosteus ; it consists of two sacs, which are cylindrical and unequal, but there are no internal 

 cells in the bladder representing lung structure. There is, however, a duct from the two lobes 

 opening into the oesophagus, and the opening is defended by a circular muscle. There are three and 



* Genus Polypterus. 



