ECHINOIDEA 275 



is further known from S.W. of Iceland, the Bay of Biscay, and 

 off the Canaries. Bathvmetrical distribution ca. 750-1800 m. 



Fig. 154. — Poriocidaris j)urpurata. Somewhat reduced 



It has also been recorded from the Indian Ocean, but only a 

 single, very young specimen. It is scarcely probable that it 

 is really the same species. 



II. Order. Diadematoidea 



(Syn. Centrechinoidea.) 



Shape of the test regular, round, spherical or flattened. Anal 

 opening in the middle of the apical area. Peristome with one pair 

 or a series of plates in each radius, while the interambulacral 

 plates never form regular series over the peristome. Gills and, 

 usually, gill-cuts present at the peristomial edge of test. Ambula- 

 cral plates mostly compound, more rarely simple. Usually both 

 ambulacral and interambulacral plates with a primary tubercle 

 (spine), and often some of the secondary tubercles (spines) grow 

 to the same size as the primary and form regular vertical or 

 horizontal series. Primary tubercles perforate or imj^erf orate, 

 crenulate or smooth. Primary spines only exceptionally with a 



