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and numerous, and fertilisation usually external. The young 

 when hatched are immature ; and there is generally a prolonged 

 larval period. 



Order CrossopterygiL Paired fins are lobate, and fringed with 

 lin-rays. Tail protocercal (diphycercal). Skin covered with 

 rhombic " ganoid " scales set with denticles. Spiracles, a 

 bilobed air-bladder (hydrostatic and respiratory), a spiral 

 valve in intestine, an optic chiasma, and a conus. 



Examples, Folypterus. Dorsal fin composed of 

 numerous finlets. Rivers of North 

 and Equatorial Africa. 

 Calamichthys. Rivers of West Africa. 



Order Actinoptervgy. Paired fins have short basal pieces, and 

 are chiefly supported by fin- rays. An air-bladder (hydro- 

 static and respiratory), a spiral valre in intestine (in the 

 more primitive forms), an optic chiasma, and a conus. 



Examples, Acipenscr (Sturgeon). Skin bears 

 keeled bony scutes. Tail hetero- 

 cercal. 

 Amia (Bow-fin). Skin has thin cycloid 



scales. 



Lepidosteus (Bony Pike). Skin covered 

 with " ganoid " scales. 



Sub-order Teleostei (Modern Bony Fishes). Skeleton 

 extensively bony ; the skull (a chondrocranium largely 

 supplemented and replaced by bone) has a large supra- 

 occipital, and is amphistylic. The vertebrae are 

 usually amphicoelous. Tail, generally homocercal, is 

 in some forms (e.g., Eels) protocercal. Skin covered 

 with flat and thin (cycloid* or ctenoid) scales. Gills 

 comb-like. An air- or swim-bladder usually present. 

 Intestine without a spiral valve. Gut, and renal and 

 genital ducts open directly to exterior, not through a 

 cloaca. The genital ducts generally continuous with 

 the gonads. Optic nerves decussate, but not forming 

 a chiasma. Without a distinct conus ; but base of 

 ventral aorta forms a non-contractile bulbus artcriosus. 

 See also general features of Teleostomi (stated above). 

 * If the hind border is smoothly rounded, the scale is tor mod 

 cycloid ; if toothed like a comb, ctenoid. 



{Examples, riupea (Herring). 

 Salmo (Salmon, Trout). 

 Gymnotus (Electric Eel). Electric 

 organ situated ventrally in tail. 

 Anguilla (Common Eel). The trans- 

 parent larva, Leptocephalus, is mar- 



