178 



VERTEBRATE ZOOLOGY 



csecilians, the tail, formed fully in the larvse, is secondarily resorbed 

 during metamorphosis. 



The change from paired fins to paired limbs is not so radical as 

 it was once supposed. Thanks to the discovery of the limb-skeleton 



Fig. 103. — Pectoral fin of extinct crossopterygian, Saurvpterus laylori. cl, 

 clavicle; co, coracoid; H, humerus; B, radius; Sc, scapula; Scl, supra clavicle; 

 u, ulna. (From Lull, after Gregory.) 



of some of the early crossop- 

 terygians, e. g. the pectoral 

 fin of Sauripterus taylori 

 from the Upper Devonian 

 (Fig. 103), it is not difficult 

 to see how a fin could be- 

 come a foot. Note that 

 this fin, minus the fringe, 

 is a hand-like structure, 

 with humerus, radius, and 

 ulna, wrist, and several fin- 

 gers. The shoulder girdle 

 of Sauripterus is also part 

 for part homologous with 

 that of an amphibian. 

 Some changes in relative 

 sizes of the elements and a 

 reduction in the number of 

 repeated parts would give 



Fig. 104.— Development of the hind foot of a salamander, Triton fceniat^ts. 

 1-6, first to fifth digits; A-F, seven stages from the simple limb-bud to the defin- 

 itive foot. (From Lull, after Eabl.) 



