66 



EXTINCT MONSTERS 



at the end of the upper jaw-bone. There are also passages in the 

 skull leading from the nostrils to the palate, along which currents 

 of air passed on their way to the lungs. Being air-breathers, they 

 would be compelled occasionally to seek the surface of the sea, in 

 order to obtain a fresh supply of the life-giving element oxygen ; 

 but, being cold-blooded and with a small brain, needing a much 

 less supply of oxygen for its work, the fish-lizards had, like fishes, 



FIG. 9. (A) Lateral and (B) profile views of a tooth of Ichthyosaurus platyodon 

 (Conybeare), Lower Lias, Lyme Eegis, Dorsetshire, (c) Tooth of Ichthyosaurus 

 communis (Conybeare), Lower Lias, Lyme Regis, Dorset. 



this advantage over whales, which are warm-blooded that their 

 stern-propeller, or tail-fin, could take the form best adapted for a 

 swift, straightforward course through the water. 



In the whale tribe the tail-fin is horizontal ; and this is so on 

 account of their need, as large-brained, warm-blooded air-breathers, 

 of speedy access to the atmospheric air. Were it otherwise, they 

 would not have the means of rising with sufficient rapidity to the 



