76 



EVOLUTION AND GENETICS 



pectoral and pelvic fins and the girdles to which they are attached 

 do not require the strength and rigidity of supporting structures. 

 In the primitive elasmobranchs the pectoral girdle consists of a 

 V-shaped piece of cartilage (Fig. 50) to which is articulated a 

 large basal cartilage of the fins, the mesopterygium. To the 

 mesopterygium are attached two other pieces, the propterygium 

 and metapterygium. These basal cartilages, of which only one 

 is found in the pelvic fins, bear a fan-like series of radial cartilages 

 to which the thin terminal portion of the fin is attached. The 

 V-shaped cartilage obviously consists of the ventral portion that 



Fig. 51. — Right pectoral fin of Sauripterus laylori from the Upper Devonian, 

 cl, cleithrum; co, coracoid; cv, clavicle; sc, scapula; s.cl., supracleithrum; 

 H, humerus; R, radius; U, ulna. (From Lull, after W. K. Gregory.) 



runs between the fins and a dorsal portion extending dorsad from 

 each fin. Each dorsal piece is surmounted by another small 

 cartilage. The ventral part is called the coracoid, the dorsal piece 

 the scapula, and the ultimate dorsal cartilage the suprascapula. 



Transitional Fins. Homologies of the structures described in 

 the preceding paragraph are not entirely clear, but in one group 

 of bony fishes, the Crosso-pterygii, a significant modification of 

 structure and use of the pectoral fins is found. These lobe-finned 

 ganoids have the strange habit of resting on the bottom of the 

 water in which they live, and supporting themselves by the front 

 fins in a manner distinctly similar to the use of the front legs 

 of the terrestrial organisms. 



The fins of some extinct forms of Crossopterygii are well pre- 

 served, and in one of these, Sauripterus taylori, from the Upper 

 Devonian, the entire skeletal structure of the pectoral girdle and 

 fin is shown (Fig. 51). In this species it is at once evident that 



