66 EXTINGT 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, 
Fic. 9.—(a) Lateral and (8) profile views of a tooth of Ichthyosaurus platyodon 
(Conybeare), Lower Lias, Lyme Regis, 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 
