70 FISH GALLERY. 



POLYPTEROIDES. 



The suborder Polypteroides includes the only modern Crosso- 

 pterygian fishes, namely Polypterus (193, and fig. 40), and 

 Calamichthys (195), fresh-water fishes of tropical Africa. The 

 dorsal fin has the form of numerous finlets, each with a spine in 

 front ; the outline of the tail is symmetrical, and the axis is not 

 up tilted in the adult, although it is in the young.. The pectoral 

 fins are obtusely lobate; the skeleton of each consists of three 

 basal elements articulating with the shoulder girdle, a row of rod- 

 like bones radiating from their periphery, and long, thin, closely-set 

 dermal fin-rays supporting the marginal fringe of the fin ; the 

 pelvic fins are not lobate. The centra of the vertebras are ossified, 

 and are concave in front and behind (amphicoelous) . The scales 

 are rhombic, set in oblique lines, thick, with an external layer of 

 smooth, hard vitro-dentine or ganoin, covered by soft skin. The 

 teeth are simple, the walls not folded. There are two vomerine 

 bones, and two jugular plates. The nostrils are on the upper side 

 of the snout and project as short tubes; the spiracles remain 

 open. 



& £ £ A # 



Fig. 40. — Polypterus bichir. 



Polypterus (193), like the Lung-fishes, is capable of breathing 

 air, but it cannot remain alive out of water more than three hours. 

 The air-bladder is double and cellular, and its duct opens into 

 the ventral wall of the pharynx. Its blood-supply is from T ,the 

 efferent vessel of the last gill ; its vein joins the hepatic vein, and 

 does not carry the blood direct to the heart as it does in the Dipnoi. 

 The young Polypterus has an external gill, which is attached 

 to the operculum (fig. 41). Ten or more species of Polypterus 

 are known ; the earliest known species is Polypterus bichir of the 

 Nile, which is said to attain a length of four feet. It feeds on 

 small fishes, frogs, and crustaceans. Calamichthys (195), is a 



