436 



CHORD ATA. 



The paired fins show similar modifications. The archi- 

 pterygium is found in the Dipnoi and consists of a median 

 axis with symmetrical lateral rays {cf. the protocercal tail). 

 The ichthyopterygium consists of one or more basal parts 

 bearing secondary rays only on the outer border. In the 

 pectoral fin there are commonly three primary basal pieces 

 {cf, skate) and in the pelvic only one. 



Fishes are usually covered or protected with scales^ of 

 which there are usually distinguished four kinds. The 

 placoid scale has a bony base and bears a spine, usually 



Fig. 311. — Fins of Fishes. 



A B 



Basipterygium. 



A, Archipterygii 



B,^ Uniserial type of ichthypterygium, 

 pelvic fin of Skate. 



found in Elasmohranchii ; the cycloid is a flat circular plate 

 arising in the dermis : with its ally, the ctenoid^ which has a 

 toothed edge, it is found chiefly in the Teleostei, Lastly, 

 the ganoid scale is a hard rhomboidal plate closely apposed 

 to its neighbours and occurring in the certain archaic fishes 

 of the Teleostomi. 



All fishes have gills borne upon four or more gill-slits. 

 The slits may be widely apart and opening separately, as 

 in Elasmobranchii, or they may be close together and 

 covered by a flap or operculum, as in the other orders. 



