550 PISCES FISHES. 



Order i. CROSSOPTERYGII. 



Ancient forms with pectoral fins obtusely lobate and uniserial (?) or 

 acutely lobate and biserial (?) ; with scales and dermal skull bones 

 often covered with enamel-like ganoin ; with a pair of jugular plates. 

 All are extinct except Polypterus and Calamoichthys from African 

 rivers. Examples, Ostcolepis (Lower Devonian), Holoptychius (De- 

 vonian), Megalichthys (Carboniferous). 



In Polypterus, the body is covered with rhombic ganoid scales, 

 there are numerous dorsal fins, the tail is diphycercal, the pectoral fin 

 has three basal pieces as in Elasmobranchs ; the air-bladder is double 

 and is used in respiration, its duct opens ventrally into the pharynx ; 

 the young form has an external gill on the operculum ; the oral part of 

 the hypophysis retains its opening into the mouth. This form may 

 almost be called a living fossil. (See Note, p. 551.) 



The following three orders are often grouped as Actino- 

 pterygii, with the following characters. The paired fins are 

 never lobate, they have short basal pieces, and are mainly 

 supported by dermal fin-rays. 



Order 2. CHONDROSTEI with cartilaginous 

 internal skeleton. (See Note, p. 551.) 



Living examples : Sturgeon (Acipenser), Polyodon, Sca- 

 phirhynchus. 



FIG. 267. Sturgeon (Acipenser sturio]. 



Note the elongated snout, the barbules bounding the ventral 

 mouth, the operculum covering the gills, the rows of bony 

 scutes, the markedly heterocercal tail. 



Extinct examples : Chetrolepis, Palaoniseus, Chondrosteus. 



In the sturgeon (Acipenser} the skin bears five rows of large 

 bony scutes ; the tail is heterocercal ; the notochord is unsegmented. 

 A snout, with pendent barbules, extends in front of the ventral mouth, 

 which is rounded and toothless. Sturgeons feed on other fishes, which 

 they swallow whole. They are the largest fresh- water fishes, for A. 

 sturio may attain a length of 18 ft. and a weight of 600 lb., while the 



