DRAGONS OF THE DEEP. 201 



surface for air or down into the deep for food. 

 The vertical tail of the Eep tiles now in question 

 shows that the need for fresh supplies of air was 

 not of such importance ; they could exist for a 

 much longer time without inconvenience. Con- 

 sequently a vertical tail would be advantageous, 

 since its undulations would serve to drive the 

 body forward through the water, as in the fish, 

 and thus facilitate the capture of prey. 



Turning to the limbs, we note again a differ- 

 ence from the whale, inasmuch as all four were 

 present instead of the fore-limbs only, though 

 the hind-limbs were in many cases extremely 

 reduced. From the walking limbs from which 

 they were evolved, they differ profoundly, being 

 now reduced to the condition of paddles, effective 

 as swimming organs, but of little use on land. 

 The skeleton of these paddles will well repay 

 brief study, for whilst they resemble the similar 

 organs of the whale tribe, and that ancient croco- 

 dile the Geosaurus, they still more closely approach 

 those of the Plesiosaurs. The paddles of these 

 old Fish-lizards, however, differ from all others, 

 in the greater specialisation which they dis- 

 play: they appear indeed to have reached the 

 maximum development ever attained by a 

 paddle. In the earliest known members of the 

 tribe the species of the genus Mixosaurus from 

 the Triassic formations, we find the simplest 

 form of the Ichthyosaurian paddle. Herein 

 the humerus or arm bone and the bones of the 

 forearm were relatively long, the latter being 

 readily distinguished from the first row of wrist- 

 bones. But in the later forms, as will be seen in 



