NO. 1 1 CLARK: A NEW GENUS OF SEA-URCHIN 175 



long and .12 mm. in diameter through the stout basal half. The stalks 

 are .10 to .18 mm. long. The valves become narrower abruptly, just 

 beyond the middle. 



In life, Idiobryssus was white, very similar to the color of the sand 

 in which it was living. In preservation and ultimate drying, the spines 

 have taken on a slight tint of cream-color and at the base are often 

 quite brownish. The minute spines of the fascioles are very distinctly 

 brownish. The dried muscular tissue at the base of the larger spines, 

 and the heads of the pedicellariae are quite brown, and the dorsal side 

 of the test has a faintly brownish tinge. 



The paratype is distinctly smaller than the holotype, about 11 mm. 

 long, rather more than 8 mm. wide and fully 4 mm. high. The fascioles 

 are very distinct and the peripetalous seems to be relatively a little 

 larger. But in all essentials, this smaller specimen is like the larger one 

 and is convincing evidence that the unusual form of the test is not 

 accidental but is a distinctive character of this odd spatangid. 



As for the relationships of Idiobryssus, there can be no doubt that 

 it is rather sharply set off from the other Spatangidae by the nearly 

 circular, scarcely depressed peristome. This suggests at once the juvenal 

 condition of Abatus as figured by Mortensen (1910, Echinoidea of 

 Swedish South Polar Expedition, pi. 9, fig. 19) but when Abatus is 

 12 mm. long, the peristome has assumed its short, wide, curved form. 

 There is no indication whatever in Idiobryssus that the peristome might 

 ultimately assume such a shape. In spite of this remarkable oral area, 

 the new genus is best referred to the Spatangidae and is apparently as 

 near Rhynobrissus as it is to any Recent genus. It may be diagnosed 

 thus: Spatangidae with concave abactinal surface, a dorsally exposed 

 periproct, no anal branches to the siibanal fasciole and a nearly circular, 

 scarcely depressed peristome. It is of course greatly to be hoped that 

 additional and adult material will soon be secured, for this spatangoid 

 is one of the most noteworthy echinoderms that the current explorations 

 of the eastern tropical Pacific Ocean have brought to light. I am very 

 grateful to Captain Hancock and to Professor Irene McCulloch for 

 the privilege of describing it without further delay. 



