A REMARKABLE NEW GENUS OF SEA-URCHIN 



( Spatangidae ) 

 (Plate 17) 



Hubert Lyman Clark 



Museum of Comparative Zoology 



On January 16, 1938, while the Velero III was at Darwin Bay, 

 Tower Island, Galapagos, two specimens of a small spatangoid were 

 dredged which proved to be of exceptional interest. They were taken 

 at Station 783-38 on a bottom of white sand and rock, in 40-70 fms. 

 The small size and the complete absence of genital pores indicate that 

 they are very young but their conspicuous peculiarities leave no doubt 

 that they represent a hitherto unknown genus for which I have selected 

 the name Idiobryssus {\hio^=^ peculiar , distinct -\-^QV(5(50z,=^th.t name of 

 an Aristotelian sea-urchin almost universally transliterated incorrectly 

 as brissus). For the species name coelus (xodo;= hollowed) is chosen 

 in reference to the concave dorsal surface. 



The holotype of 



Idiobryssus coelus 

 is 12 mm. long, 10 mm. wide, just back of petals II and IV, 4 mm. 

 high at center of abactinal system, about 4.5 mm. anteriorly and 5 mm. 

 posteriorly; these high points are midway between the abactinal system 

 and the anterior and posterior ends of the test. The upper surface 

 (figure 1) is thus very definitely concave and the lower surface is cor- 

 respondingly convex. This unusual and very noticeable form is clearly 

 shown in figure 2. Abactinal system (it can scarcely be called "apical" 

 since it is at the bottom of a notable depression!) at the approximate 

 center of the upper concave surface but anterior to the actual middle 

 of the test which projects considerably below and beyond the periproct; 

 no distinct sutures can be made out between the plates ; there are no 

 genital pores but what are apparently ocular pores seem to be present 

 and there are 5 or 6 madreporic pores in a small indistinct group. 



Ambulacrum III not at all depressed and so ill-defined its exact 

 limits cannot easily be made out. Petals I and V somewhat elliptical, 



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