﻿MARSHALL ISLANDS ECHINODERMS — CLARK 281 



Bikini Island; 30 feet; coral and foraminiferal sand bottom; Mor- 

 rison, April 23, 1947. One very young specimen, U.S.N.M. No. E. 

 7506. 



Notes. — The very small specimen (E. 7506) bears little resemblance 

 to the adult, but instead suggests a small slender spined Echinometra, 

 It is slightly oval, two-thirds as broad as long, and twice as long 

 as high, with the aboral surface gently convex. It measures 7 mm. 

 in length, 5.5 mm. in width, and 3.5 mm. in height. 



The genitals are fused into a somewhat irregular five-sided plate 

 1.5 mm. long and 1 mm. broad. The posterior genital appears to be 

 much larger than the others and bears a large perforated tubercle 

 surrounded by a broad areole. Numerous other tubercles about half 

 as large are scattered over the surface of the combined genitals. 

 There are four large madreporic pores, each on the summit of a short 

 tube, the four tubes crowded into a square with rounded angles and 

 slight notches on the sides. No pores are visible in the genital plates. 



The triangular oculars are very small, each situated on one of the 

 sides of the combined genitals. 



There are from 10 to 12 large interambulacrals in each column. 

 These are at first hexagonal, toward the ambitus gradually becoming 

 transversely elongated, and at the ambitus about twice as broad as 

 long; below the ambitus they gradually become shorter again. 

 Aborally each interambulacral plate bears a large central perforated 

 tubercle surrounded by a broad areole margined by a circlet of small 

 and glassy, usually contiguous, tubercles. On the broad interambu- 

 lacral plates at and below the ambitus there are commonly two tu- 

 bercles to a plate. 



The ambulacrals are at first minute. After a series varying from 

 12 to 19, much larger ones, each with a large central tubercle, appear, 

 and soon all are large with usually three pores in a slightly curved 

 line along the outer border. At the ambitus the ambulacrals are 

 hexagonal and about as high as the much broader interambulacrals, 

 with central tubercles about as broad as those of the latter. On the 

 aboral surface the very small ambulacrals always extend farther 

 down on one side of the ambulacral areas than on the other. 



There is a single glassy spheridium situated in a deep and capacious 

 pit at the adoral end of each ambulacrum. 



The peristome is circular, 2 mm. in diameter, densely and evenly 

 covered with very minute spinous plates. The mouth is closed and 

 no teeth are visible. 



The periproct is large, situated just below the ambitus and sloping 

 slightly inward, diamond shaped with rounded angles, transversely 

 elongated, 2.3 mm. wide and 1.4 mm. high. Most of its surface is 

 covered by three large plates of which the two outer reach halfway 



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