OPHIACTIS SIMPLEX. 



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adult ; under arm-plates pentagonal, with an angle inward ; longer than 

 broad, nearly separated by side arm-plates ; two full arm-joints admitted 

 into the disk. The proportions of tlie disk to the arms may be as fol- 

 lows : 1.2:4.2, l.G : G, 2.5:15, 4:28. The color of the young, when 

 dry, is tawny brown, the arms being barred with greenish brown ; as 

 they grow larger, the tendency of tint is more towards greenish. Dr. 

 LeConte says the color of the living animal is '' pale beneath, a])Ove 

 blackish brown ; arms marked with a few paler bands." He adds, 

 that "the scales are all finely granulated" (spinous), so that, in the 

 dried specimens, these spines have projjably for the most part been 

 rubbed off. 



It is probable that the adult of this species has always five arms, 

 while the young have six. It is easily distinguished by its small radial 

 shields, which, together Avith the form of its arm-spines and upper arm- 

 plates, place it nearer to Amphhira than are the other species of 

 Ophiactis. I feel pretty sure that 0. Orsfedii, of which Dr. Liitken got 

 one specimen, through Dr. Orsted, from Puntarenas, is the same as this 

 species. The only noteworthy difference is in the shortness of the 

 arms, which were apparently broken in the above-mentioned specimen, 

 and could not therefore be accurately measured. It is also possible 

 that 0. arenosa Ltk. will fall among the synonymes of 0. simplex. But 

 these questions must be answered by future observers. 



LIST OF SPECIMENS, 



