Boone, Echinoidea, Cruise of "Alva" 1931 251 



Order: DIADEMATOIDA 



Suborder: Aulodonta 

 Family: CENTRECHINIDAE 



Genus: ECHINOTHRIX Peters 

 Ekhinothrix diadema Linne 



Plates 83 and 84 



Types Linne's type came from the seas of India. It is de- 

 posited at Upsala. 



Distribution: This interesting urchin is also a widely dis- 

 tributed member of the littoral fauna of the Indo-Pacific, having 

 been many times reliably reported from the Red Sea in the north 

 to Mozambique, East Africa, in the south, eastward at Mer, Tor- 

 res Straits, and the Hawaiian Islands, Society Islands and the 

 Paumotu Archipelago. 



Material examined: One large blackish specimen, from Te- 

 viatoa Reef, Raiatea Island, Society Islands, August 21, 1931. 

 One very young specimen from the same locality. 



Discussion: The large specimen has a height of 50 mm., 

 width of 90 mm., and length of 90 mm.; the small specimen 

 has a height of 5 mm., width of 8 mm., and length of 9 mm. 

 This young specimen has the primaries alternately banded with 

 five bands of purplish brown, including the base and tip, with 

 four alternating bands of lavenderish-creamy (preserved speci- 

 men) . In this specimen the texture of the primaries appears more 

 sharply etched on each separate groove than is true of the larger 

 specimen. 



The perplexing intergradation of the two species that com- 

 prise this genus has been ably discussed by Dr. H. L. Clark (1912) . 

 The structure of the test and spines of the present species is 

 well known from the excellent description of Agassiz and others. 

 Mortensen (1904) has given a fine description of the pedicel- 

 lariae. 



Echinothrix diadema has a fairly constant specific character in 

 the primary spines of the interambulacral region, these spines 

 being rather thick in transverse section, with the diameter of the 

 central cavity much less than half the diameter of the spine and 

 with the spinules of the external surface arranged in close-set 

 longitudinal series and not in definite whorls. 



