436 CHORDATA. 
The paired fins show similar modifications. The archi- 
pterygium is found in the Dzpxoi and consists of a median 
axis with symmetrical lateral rays (¢f 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 
(of. 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. 
Basipterygium. | 
A, Archipterygium. ” B, Uniserial type of ichthypterygium, 
pelvic fin of Skate. 
found in Elasmobranchit ; the cycloid is a flat circular plate 
arising in the dermis: with its ally, the ctezoid, which has a 
toothed edge, it is found chiefly in the Zeéostei, 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 Elasmobranchit, or they may be close together and 
covered by a flap or operculum, as in the other orders. 
