228 OUTLINE CLASSIFICATION OF ANIMALS 
2. Diprotodontia, 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 (Ornithorhynchus), 
and the two spiny ant-eaters (Echidna and Proechidna). 
Cuass AvES or Birps, 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. 
Cuass REpTILIA OR REPTILES, cold-blooded scaly animals 
with five living orders, and many extinct ones : 
1. Crocodilia or crocodiles and alligators. 
2. Chelonia or turtles and tortoises. 
3. Ophidia or snakes. 
4. Lacertilia or lizards. 
5. Rhynchocephalia, including only the New Zealand lizard 
called Hatteria, with many primitive characters. 
Ciass 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. 
Cuass 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 different from 
each other, but sharing with the Elasmobranchs certain primi- 
