CLARK: THE STARFISHES OF THE GENUS HELIASTER. 41 



B. Abactinal surface covered with numerous small, often sub- 

 acute, rarely capitate, spines of nearly uniform length, not 

 arranged in radiating series except on rays, where five such 



series are usually more or less evident microbrachius 



BB. Abactinal surface with rather large, often capitate spines, ar- 

 ranged in more or less distinct radial series, especially on 

 rays, where three such series are very evident. 



Abactinal spines not very numerous, 15-20 per sq. cm. 

 where thickest, more or less cylindrical, often subacute, 

 rarely distinctly capitate ; pedicellariae often wanting on 

 actinal surface ; rays often free for more than 20 per cent 

 of their length cumingii 



Abactinal spines more numerous, 25—50 per sq. cm. where 

 thickest, low, usually capitate ; pedicellariae frequent on 

 actinal surface ; rays seldom free for more than 20 per cent 

 of their length polybrachius 



Artificial Key to the Species. 



A. Rays more than 80, rarely as few as 27 or 28. 



B. Rays free for 30 per cent of their length or more ; west coast 



of South America helianthus 



BB. Rays free for less than 30 per cent of their length. 



C. Abactinal surface with very numerous small spines, 

 rarely capitate ; five subequal series on rays ; west 



coast of Mexico and Central America microbrachius 



CC. Abactimil surface with fewer, larger, capitate spines ; 

 three series on rays. 



Abactinal spines not crowded, little or not at all 



capitate ; Galapagos Islands cumingii 



Abactinal spines numerous, often crowded, especially 

 near margin of disc, usually distinctly capitate ; west 



coast of tropical South America polybrachius 



AA. Rays never more than 28. 



B. Abactinal surface of disc with spines smaller than the margi- 

 nal series on rays; diameter of adult 80-120 mm.; Juan 



Ferdaudez canopus 



BB. Abactinal surface of disc with large, often capitate spines ; 

 diameter of adult 110-180 mm. 



Rays free for 40-55 per cent of their length ; west coast of 



Mexico and Central America kubiniji 



Rays free for 50-70 per cent of their length ; Galapagos 

 Islands multiradiatus 



