TELEOSTOML 569 



They are usually rather small fishes, with minute rhomboidal shagreen- 

 like scales, and a strong spine in front of each fin, except the caudal. 

 In some genera (Parexus, Climatius] there are two rows of small 

 intermediate spines between the proper pectorals and the pelvics. 



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FIG. 303. Outline of Acanthodes sulcatus. After Traquair. 

 /., Pectoral fins ; z>., pelvics ; ., anal ; d., dorsal. 



Sub-Class II. TELEOSTOMI 



Fishes with more or less ossified skeletons, especially as 

 regards skull, jaws, operculum, and pectoral girdle. The 

 skull is hyostylic, the jaws being supported by the hyoman- 

 dibular. The pelvic girdles are usually rudimentary or 

 absent. The mouth is usually terminal ; the scales are in 

 the majority soft and cycloid. There is always a gill- 

 cover; the inter-branchial septa are much reduced; the 

 gill-filaments project freely from the gill-arches. There is 

 usually a swim-bladder. There are no claspers, no naso- 

 buccal grooves; there is no cloaca. The fore-brain has 

 a non-nervous roof. The ova are small and numerous, 

 usually meroblastic, sometimes holoblastic. Fertilisation is 

 usually external. 



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 between 

 the rami of the lower jaw. All are extinct except Polypterus and 

 Calamoichthys from African rivers. Examples, Ostcolepis (Lower 

 Devonian), Holoptychius (Devonian), 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, then two rows of 

 radials, and then the dermal fin-rays or dermotrichia ; 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 



