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PACIFIC SCIENCE, Vol. XVII, July 1963 
the periphery of 16 islets completely, and 5 
others partially, thus covering 67% of the sea- 
ward shores of all islets in the atoll. The islets 
are numbered clockwise on the map of Rongelap 
Atoll shown in Figure 1 starting at the north- 
west extremity. Only about 2 % of the inter-islet 
reef area and shallower lagoon bottom regions 
supposedly suitable for H. atra was censused. 
Distances were measured by chain on the 
seaward shores of four islets, Aniiru, Kabelle, 
Namoen, and Rongelap, while at other islets 
distances were either determined by pacing, or, 
in most cases, estimated. On the east rim of the 
atoll high-altitude vertical, and elsewhere low- 
altitude oblique, aerial photos were employed to 
aid in orientation. Photographs taken on the 
reef-flat of undisturbed sea cucumbers were 
sometimes used in the estimations of abundance. 
The places of concealment under overhanging 
ledges were inspected, but rocks were seldom 
lifted to check for specimens. 
Table 1 shows the distribution of H. atra and 
H. leucospilota at the localities surveyed. Section 
numbers representing 100-m units of seaward 
shoreline appear under the appropriate fre- 
quency group. The central frequency groups in- 
crease by a factor of 10 while smaller ranges of 
numbers delimit the extremes. Along with the 
range in the column headings are the means in 
parentheses— geometric except for the first, 
where the zero limit precludes use of the geo- 
metric mean. Geometric and arithmetic means 
were tested for agreement with totals of the 
last two columns showing sums by localities 
taken directly from a previous tabulation, which 
also supplied the data for the body of Table 1. 
