228 OUTLINE CLASSIFICATION OF ANIMALS 



2. Difrotodontia, herbivorous forms, with only two incisor 

 teeth in the lower jaw, including kangaroos, the wombat, &c. 



Sub-class III, Prototheria, egg-laying primitive mammals, 

 with one living order, Monotremata, which contains only three 

 living kinds of mammals, the duck-mole (Omithorhynchus), 

 and the two spiny ant-eaters (Echidna and Proechidna). 



Class Avbs or Birds, including warm-blooded forms 

 clothed with feathers. The living forms may be divided into 

 two divisions : 



1 . Carinatae, or flying birds, with many orders, having a keel 

 on the sternum, and usually the power of flight. 



2. Ratitae, or running birds, long-legged forms, with no keel 

 on the sternum and rudimentary fore-limbs, including only 

 a few living forms, e. g. the ostrich, cassowary, emu, &c. 



Class Eeptilia or Eeptilbs, cold-blooded scaly animals 

 with five living orders, and many extinct ones : 



1. Crocodilia or crocodiles and alligators. 



2. Ghdonia or turtles and tortoises. 



3. Ophidia or snakes. 



4. LacertiUa or lizards. 



5. Rhynchocefhalia, including only the New Zealand lizard 

 called Hatteria, with many primitive characters. 



Class Amphibia or Amphibians, smooth-skinned forms with 

 gills in the early stages, and lungs in the later, including forms 

 like frogs and toads, without tails in adult life, and those like 

 newts and salamanders, which keep the tail throughout life. 



Class Pisces or Fishes, permanently aquatic forms with 

 gills and scales. The classification is a matter of difficulty, 

 owing to the vast number of fossil forms. It is sufficient to 

 note here that most living fishes are Teleosteans, forms with 

 a completely bony skeleton and a swim-bladder used as hydro- 

 static organ. Sharks and skates, &c., are Elasmobranchs, and 

 have a cartilaginous skeleton, the living forms being the 

 specialized remnants of a large and primitive group. As 

 Ganoids we may group a few living forms, very difierent from 

 each other, but sharing with the Elasmobranchs certain primi- 



