THE ANIMAL KINGDOM 23 



2. OsTRACODERMi. Fossil monorhine fishes related to the cyclostomes. 



3. Pisces. With paired appendages and movable lower jaw. Skin 

 usually scaly. Permanent gills. Fishes are subdivided into five subclasses : 



Elasmobranchii. Gills lack operculum. Skeleton cartilaginous. 

 Sharks, skates, and rays. 



Crossopterygii. Fossil forms related to the ganoids. 



Ganoidei. With operculum. Cartilage skeleton largely replaced by 

 bone. Garpike, sturgeon. 



Teleostei. With operculum and bony skeleton. Common bony fishes. 



Dipnoi. With gills and one or two lungs. 



All following are Tetrapods. 



4. Amphibia. Living forms are without scales and usually have lungs. 

 Toed appendages instead of fins. Claws or nails lacking. 



The Stegocephala are a group of fossil amphibians. 

 Fishes and Amphibia grouped together as Ichthyopsida. 



All to this point are Anamnia. All that follow are Amniota, having the 

 embryo protected by an amnion. 



5. Reptilia. Adults scaly. Lungs only — no gills. Aortic arch on 

 both sides. The Theromorpha are fossil reptiles. 



6. AvES. Feathered. Modern forms toothless. Aortic arch on 

 the right side only. 



Reptiles and birds grouped together as Sauropsida. 



7. Mammalia. With mammary glands and hair. 



Sub-Classes of Mammals 



1. Monotremata. Egg-laying mammals. 



2. Marsupialia. Pouched mammals. 



3. Placentalia. Mammals with a placenta. 



Orders of Placentalia 



1. Insectivora. Insect eaters. 



2. Xenarthra. Toothless or teeth without enamel. 



3. Rodentia. Incisors specialized for gnawing. 



4. Carnivora. Flesh eaters. 



5. Artiodactyla. Generally even number of hoofs. 



6. Perissodactyla. Generally odd number of hoofs. 



7. SuBUNGULATA. Proboscidians, hyrax, and sirenians. 



8. Cetacea. Whales. 



9. Chiroptera. Winged mammals. 



10. Primates. Usually a single pair of thoracic mammae. Thumb 

 usually opposable. 



