54 



ANIMAL LIFE PAST AND PRESENT. 



If, however, the Turtles have 

 been specially modified for an 

 aquatic existence, still more 

 markedly is this the case with 

 the extinct Tchthyosaurs or Fish- 

 Lizards (Fig. 17). As the struc- 

 ture of these reptiles is fully 

 described in a later chapter, we 

 need only allude here to the 

 peculiar pavement-like structure 

 of the bones of the paddles ; this 

 being the extreme modification 

 which limbs have undergone for 

 the purposes of an aquatic life. 

 Since, as shown by the absence 

 of gills and the indications of 

 remnants of a horny covering to 

 the body, there is abundant evi- 

 dence that the Ichthyosaurs are 

 descended from land animals, 

 they occupy a position among 

 reptiles precisely analogous to 

 that held among Mammals by 

 the Whales of the present epoch. 



The long-necked Plesiosaurs, 

 to which a special chapter is 

 likewise devoted, were reptiles 

 equally well adapted for an 

 aquatic existence, but in which 

 the modification of the limbs 



