232 



PAL/EONTOLOGY 



They underwent complete adaptation to a marine life, 

 and approximated to fishes in shape and general structure, 

 but in accordance with the law of irreversibility in 

 evolution they could not lay aside all the characters they 

 had gained during their ancestral land-life, nor could they 

 recover certain fish-structures which their land ancestors 

 had lost the gills in particular. If we compare the 

 limbs (or paddles) of an ichthyosaur with the fin of 

 Ceratodus on the one hand and the limb of the salamander 



N 



FIG. 66. FINS AND LIMBS. 



a, Ceratodus ; b, a newt ; c, Ichthyosaurus. The dotted lines connect 

 homologous structures. 



on the other, we see that while in outline, absence of 

 separate fingers, and slight possibility of movement 

 between the numerous small, closely -packed bones it 

 agrees with the fish, its fundamental structure is that of 

 a land-animal (Fig. 66, c). So, too, with the tail-fin : 

 externally it resembles the homocercal tail of a bony 

 fish, but the end of the vertebral column bends down 

 into the lower lobe (Fig, 65, c). In these and other ways 

 ichthyosaurs show convergence towards fishes, but the 

 resemblances are not exact and are mainly external. 



