Indo-Padfic Echinoderms — Clark 
245 
urchins are the two species of slate-pencil or 
cigar urchins of the genus Heterocentrotus. One 
of these, H. mammillatus , occurs from the Red 
Sea and Tanganyika, Madagascar, Mauritius, 
and Rodriguez to the Cocos-Keeling Islands, 
northwestern Australia, New Guinea, Torres 
Strait, the Philippines, New Caledonia, and 
Fiji, and also in the Mariana, Bonin, Ryukyu, 
and Hawaiian Islands. It appears to be absent 
from the central Polynesian region except for 
Johnston Island and the Tuamotus, where it 
was recently found by Dr. Morrison. 
The other more specialized species, H. tri- 
gonarius, occurs at Zanzibar, Natal, Madagas- 
car, Mauritius, Rodriguez, Java, the Philip- 
pines, Tonga, Samoa, and throughout 
Polynesia. 
Both species occur at Madagascar, Mauri- 
tius, Rodriguez, the Philippines, the Tuamo- 
tus, and Johnston Island. 
As both species have apparently the same 
habits, living normally in holes and crevices 
in the reefs and sometimes together in the 
same group, the reason for the difference in 
distribution in the extreme western and in the 
eastern part of their ranges is obscure. It may, 
of course, be due in part to insufficient knowl- 
edge of their distribution among the atolls; 
H. mammillatus may be more generally dis- 
tributed here than the records available at 
present indicate. 
It appears somewhat paradoxical that, al- 
though large species and large individuals for 
the most part do not extend into the groups 
of small Polynesian islands, a few species 
reach their maximum size in this area, on the 
northeastern periphery of their range. The 
largest specimen of Heterocentrotus trigonarlus 
I have seen is from Johnston Island and 
measures 123 by 100 millimeters with a height 
of 68 millimeters and with the longest spines 
150 millimeters. In another specimen, pos- 
sibly larger, the longest spines are 165 milli- 
meters long. Some from Bikini are almost as 
large (Clark, 1949: 71). The largest known 
specimens of Brissus latecarinatus are those 
from the Marshall Islands recorded in the 
following pages. 
In the Atlantic, Echinometra lucunter reaches 
its maximum size on the northern and south- 
ern limits of its range in Bermuda and Brazil 
(Clark, 1933: 83), and Brissus brissus in the 
Mediterranean reaches nearly twice the size 
that it does in the Caribbean {ibid., p. 91). 
The largest known specimen of Linckia guild- 
ingii, with a radius of 215 millimeters, is from 
Bermuda. 
From the zoogeographical and historical 
points of view, the most interesting and sig- 
nificant echinoderms are not to be found in 
the warm and brilliantly illuminated tropical 
littoral, but in the dimly illuminated and 
cooler zones from 5 or 6 fathoms downward 
to the depth, which differs in different areas, 
where a localized fauna, if present, begins to 
merge into the increasingly widespread abys- 
sal fauna. That such an intermediate fauna 
may be of much significance is indicated by 
the genus Psychocidaris, the only representative 
of the family Psychocidaridae, related to the 
Cretaceous Tylocidaris, known only from the 
Bonin Islands in about 100 fathoms. 
COLLECTION DATA 
Onotoa Atoll, Gilbert Islands 
The localities in the Onotoa Atoll, Gilbert 
Islands, are listed by numbers. The data for 
each numbered locality are as follows. 
G.O.C.-24. Toward the southern end of a 
lee reef stretch known as Rakai Ati, in an 
area of small coral patches fairly thickly in- 
terspersed on lime sand and coral debris; the 
bottom is at depths of 3-4 feet at low tide. 
Preston E. Cloud, Jr., and D. W. Strasburg, 
July 26, 1951. 
G.O.C.-25. About 4.25 miles S.86°W. from 
Aiki Maneaba on the lagoon side of the broad 
reef passage north of a narrower passage called 
Rawa Bao, from small patch reefs rising to 
within 4-6 feet of the surface from a lime- 
sand and coral-gravel bottom at 12 feet. P. 
