INTRODUCTION. 5 
Sub-class I, Elasmobranchit. 
Fishes with cartilaginous skeleton, mouth usually on the lower 
side of the head, the gills usually opening separately on the neck, and the 
tail with the upper lobe the larger (heterocercal). Sharks and skates. 
The Holocephali differ in having the gill slits covered with a fold of 
skin, so that but a single external opening appears. 
Sub-class II. Ganoidea. 
Intermediate between elasmobranchs and _ teleosts.—Garpike, 
sturgeon. 
Sub-class III. Teleostet. 
Fishes with bony skeleton, mouth with true jaws at the tip of the 
snout, gill openings concealed by an operculum or gill-cover supported 
by bone. ‘Tail with upper and lower lobes equal.—All common fishes. 
Sub-class IV. Dipnoi. 
The lung fishes are tropical forms in which the air bladder func- 
tions as a lung, the gill openings are covered with an operculum, and 
the tail is very primitive (diphycercal). 
Class II. Amphibia. 
Ichthyopsida with legs replacing the paired fins, lungs present and 
replacing the gills in the adult, nostrils connecting with the mouth. 
Sub-class I. Stegocephali. 
Extinct amphibians with well developed tail. 
Sub-class II. Urodela. 
Amphibia with well developed tail, gills sometimes retained through 
hfe.—Salamanders, Tritons, newts, efts. 
Sub-class III. Anura. 
Tailless as adults, the young a tadpole with external gills.—Frogs 
and toads. 
Sub-class IV. Gymnophiona. 
Blind, burrowing, legless amphibians occurring in the tropics.— 
Cecilians. 
