GENERAL SURVEY OF THE ANIMAL KINGDOM. 7 



Reptiles are scaly-skinned, they never breathe by gills, 

 their heart is practically four-chambered, their embryos (like 

 those of Birds and Mammals) have two birth-robes, an 

 amnion and an allantois. The extinct Saurians are very 

 numerous and diverse, and have affinities with Birds (and 

 with Mammals ?) as well as with the modern Reptiles. 



There is no doubt that Birds (Ayes) are linked to Reptiles, 

 not only by resemblances in structure, but by extinct inter- 

 mediate types. Divergent as they are in habit, they are 

 ranked together as Sauropsida, in contrast to Fishes and 

 Amphibians — Ichthyopsida — on the one hand, and to Mam- 

 mals on the other. Feathers and wings, hot blood and 

 airy bodies, characterise Birds. There are three great 

 divisions : — 



1. Saurune. Reptile-like birds — the extinct Archaopteryx, with 



feathers and wings, but also with teeth and a reptile's tdl. 



2. Ratitse. Flat-breasted running birds, e.g. , Ostrich and kiwi. 



3. Carinatje. Keel-breasted flying birds, e.g. , Pigeon and eagle. 



Finally, we reach the Mammalia, superficially charac- 

 terised by the hair on the skin, more profoundly by the milk 

 which the mothers give to their young. There are three 

 grades of mammalian excellence : — 



1. Prototheria, Ornithodelphia, or Monotremes — the egg-laying 



duckmole ( Ornithprhynchin) and Echidna. 



2. Metatheria, Didelphia, or Marsupials — the prematurely-bearing, 



usually pouch-possessing kangaroos and their relatives. 



3. Eutheria, Monodelphia, or Placentals — those in which there is 



a close (placental) union between the unborn embryo and its 

 mother. 



The placental Mammals include many orders : — 



(a) The primitive sloths and ant-eaters — Edentata. 



(b) The primitive dugong and manatee — Sirenia. 



\c) The whales (Cetacea), the hoofed Ungulata (including elephants 



and Hyrax), and the Rodentia. 

 (li) The Insectivora, the bats (Cheiroptera), and the Carnivora. 

 {e) The Lemurs, and the Primates — the monkeys and man. 

 And extinct forms which link these sets together. 



