FINS AND LIMBS 



3*5 



limb. The most convenient starting-point is the blunt lobate 

 fin of the Osteolepidoti (and primitive Dipnoi) with a single 

 large radial at its base, and an increasing number of radials 

 arranged fanwise running to the outer border of the fin. In 

 such a form as Sauripterus (one of the Osteolepidoti) the single 

 basal radial of the pectoral fin may perhaps be held to represent 

 the humerus, and the next two correspond to the radius and 

 ulna of the terrestrial fore limb. In a general way the next 

 radials represent the carpals and metacarpals. 



The earliest limbs probably had more than five fingers, 

 and the number of rows of radials in the distal part of the fins 



Fig. 160. — Ventral view of the abdominal ribs or gastralia, and pectoral 

 girdle of Sphenodon. 

 cl, clavicle ; co, coracoid ; g, gastralia ; gc, glenoid cavity ; ic, inter- 

 clavicle ; r, ribs (true) ; s, scapula. 



of Sauripterus is greater than five. But if the pectoral fin of 

 Sauripterus be compared with the arm of a primitive amphibian 

 like Eryops, it is easy to see how the structure of the latter 

 could be derived from that of the former. The evolution of 

 the five-digited, or pentadactyl limb is an adaptation to 

 locomotion on land. During this transformation, the limb- 

 girdles must have become better developed, for an animal in 

 air is relatively heavier than in the water, and the limbs are 

 subjected to greater strains and stresses. At the same time, 

 the girdles of the earliest land-vertebrates closely resemble 

 those of their aquatic ancestors. So in Eogyrinus (fossil 



