144 THREE CRUISES OF TIIE BLAKK’? 
branching coral with slender terminal twigs, while the main 
branches are spirally arranged, with a slender brilliantly irides- 
cent calcareous axis. The polyps are large, and placed rather 
far apart. It belongs to the deep-sea family of Chrysogorgidæ 
(Fig. 456), established by Verrill for such gorgonians as have 
usually an iridescent axis with spiral branches. They are among 
Fig. 450. — Chrysogorgia. g- 
the most beautiful and interesting of the gorgonians. A unique 
and striking species of the group is Iridogorgia Pourtalesu ( Fig. 
456 a) with its regular upright spiral main stem, and long, flex- 
ible, undivided branches, arranged in a single row nearly at 
right angles to the axis ; forming a broad spiral like the skeleton 
of a spiral staircase. The species are remarkable for their ele- 
