CHAP. XI 



INTRODUCTION TO TETRAPODA 



403 



Lepidosiren (Fig. 178). In the initial phases of the speciahzation of 

 such hmbs for terrestrial progression two processes would naturally be 

 especially marked : (i) the spreading out of the tip of the limb to form 

 a foot and (2), in correlation with the increasing rigidity of the supporting 



Fig. 177. 



Showing the correspondence in general form of the body in the three main subdivisions of the 

 Tetrapoda. A, an Amphibian (a Newt — Triton) ; B, a Reptile (a Lizard — Lacerla) ; C, a Mammal 

 (a thick-tailed Opossum — Didelphys cmssicauda). 



skeleton, the concentration of the at first diffuse flexibility into definite 

 localized joints. As regards the first of these it is obvious that the 

 spreading out of the foot into diverging digits with more or less inde- 

 pendent movement would make for closer fitting to the solid substratum 

 and consequently firmer support. 



As regards the establishment of joints it is of interest to note that when 



Fig. 17S. 



Young Lepidosiren, showing the flexure assumed by the hind limb when the animal is pushing 



itself along. 



young Lepidosirens are watched clambering about amongst the water- 

 weeds their still flexible hind limbs may be seen to take on a curvature 

 like that shown in Fig. 178 — a type of curvature exactly such as would 

 give rise with increasing rigidity of the supporting skeleton to joints 

 at the two points of maximum curvature corresponding to the knee and 



2 D I 



