Chœtodiadema grmmlatum n. g., n. sp., 

 a new Diadematid from the Gulf of Siam. 



By 

 TJi. Mortensen, Dr. phil. 



Among the Echiiiids which I collected in the Gulf of Siam in 

 1900 a most interesting new form of Diadematids is found, allied to 

 Astropyga, but differing from that genus and all other Diadematids 

 especially by its peculiar pore areas. — Though I am no friend of 

 preliminary reports, I think it right in this case to give a prelimi- 

 nary description of the form concerned, as it will probably take a 

 rather long time, before the final report can be published. The 

 full description with figures will appear in : The Danish Expedition 

 to Siam 1899 — 1900. Results of the Zoological Collections made 

 by Dr. Th. Mortensen. (Kgl. Danske Vidensk. Selsk. Skrifter, uaturv. 

 og raathem. Afd.) 



The test is greatly depressed and very flexible. The abactinal 

 side looks much like Astropyga. The apical area is large , but 

 the genital plates are not much elongated; inside the genital and 

 ocular plates there is a rather broad band of small, irregular plates, 

 carrying small spines. A small genital papilla is found, covering 

 the genital openings. The interambulacral areas do not on the abac- 

 tinal side differ in any essential character from those in Astropyga. 

 In the outer series of large tubercles the uppermost one is found 

 on the 9 — 10*^ plate from above, and there is no distinct series 

 of secondary tubercles on either side of this series, only irregularly 

 placed miliary tubercles. The inner series of interambulacral 



Vidcnsk. Meddel, fra den naturh. Foren. 1903. 1 



