92 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



longitudinal folds. This hypothesis is further sus- 

 tained by the acanthodean genus Climatius, where a 

 series of spines intervenes between the paired fins in 



% 



line with them. From what the Acanthodeans can 

 have been descended is at present conjectural. They 

 trace their ancestry ultimately to Branchiostoma (Am- 

 phioxus) through forms not yet discovered. This ge- 



