236 OUTLINES OP E VOLUTION AEY BIOLOGY 



the fore limbs are modified to form wings for flying in the air ; 

 and the same is true of the fore limbs of bats and of the extinct 



FIG. 91. The Duck-billed Platypus (Ornithorliynclms anatinus). (From 

 British Museum Guide.) 



flying reptiles known as Pterosauria (pterodactyls). The Cetacea 

 (whales and porpoises), the Sirenia (dugongs and manatees) and 

 the seals, amongst mammals ; and the turtles and extinct 



FIG. 92. The Echidna or Spiny Anteater (Echidna aculeata). (From British 



Museum Guide.) 



Ichthyosauria and Plesiosauria amongst reptiles, on the other 

 hand, have limbs which take the form of paddles and are 

 specially adapted for swimming in water. 



We find, then, three chief types of locomotor organs amongst 



