280 
PACIFIC SCIENCE, Vol. XI, July, 1957 
sian collections previously mentioned. In in- 
stances where these collections contain only 
one or two specimens of a species, the specific 
collecting data are presented with no attempt 
at generalization. Unless otherwise qualified, 
only data referring to specimens taken alive 
were used. Where observations by collectors 
are given, e.g., "Bayer reports,” with no cita- 
tion of a publication, the information was 
obtained by personal communication. For 
some species, additional remarks from the 
U. S. National Museum collections and from 
published accounts for other Indo-Pacific 
areas are included. Mention is made of asso- 
ciations with other mollusks or other inver- 
tebrates, whenever such associations have 
been reliably reported. 
Reef Terminology 
In the ecologic notes, an attempt is made 
to describe reef environments in "terms that 
are self-explanatory and comprehensible to 
the lay reader as well as to the trained scien- 
tist. The aim is to avoid terms that are too 
specific to apply to reefs in general, since 
detailed reef characteristics vary from island 
to island. By and large, the terminology out- 
lined by Tracey, Emery, and Cloud (1955) 
comes the nearest to that followed here; and 
for the reader’s convenience, their figure 1 is 
reproduced here, with a few emendations, as 
Plate 3. 
In these discussions, a Seaward Reef Flat 
(Tracey, et al. Reef Flat) is the upper surface 
of an ocean-facing reef. It is also called here 
an Ocean Reef Flat . A Seaward Reef Flat is 
generally broad and well- developed. Its shore- 
ward area (Tracey, et al. Inner Reef Flat) is 
usually intertidal and thus is exposed at low 
tide, commonly with rocks left standing in 
residual pools; it is covered by a few feet of 
water at high tide. Species living in this zone 
are adapted to periodic exposure or live in 
tide pools where they are always in water. Be- 
cause of its variety of habitats, this intertidal 
part of the Seaward Reef Flat is well populated 
by many species representing several different 
families. 
The seaward part of the Ocean Reef Flat 
(Tracey, et al. Outer Reef Flat) is the area 
immediately shoreward of the Reef Edge. It 
is ordinarily a zone of circulating water but 
not of heavily breaking surf; it is typically 
below low tide line and thus is covered by a 
few inches to a few feet of water at low tide. 
Species living in this zone are usually never 
exposed, even at low-low tide. This part of 
the reef flat is also well populated. 
The Seaward Beach is at the shoreward-most 
edge of the Seaward Reef Flat. It is characteris- 
tically above high tide line. Species which 
live on or near the Seaward Beach (e.g., Lit- 
torina spp.) are able to survive long periods 
of exposure to the air. Few species are so 
adapted; consequently the Seaward Beach is 
sparsely populated. However, empty shells of 
species living in other areas may be found 
here. 
The Reef Edge (Tracey, et al. Seaward Reef 
Margin) is the seaward margin of the Seaward 
Reef Flat. It is intertidal and thus is dry, except 
for spray, at low tide. The Reef Edge is exposed 
to a heavy surf. Consequently, species living 
in this habitat must be adapted to withstand 
the shock of violent wave action. Gastropods 
living on the Reef Edge commonly have a 
strong muscular foot with which they cling | 
to their precarious homesite (e.g., Patella j 
stellaeformis, Trochus maculatus) . Other workers 
have called the Reef Edge a reef crest, coralline 
ridge, algal ridge, and Lithothamnion ridge, j 
Elongate surge channels or shallow passes 1 
indent the Reef Edge. 
Plate 1 . Truncated and flourishing seaward reef flats. Above: Rock surface with patchy thin algal veneer backed 
at right by beach of alternating loose sand (light) and pitted coral-algal limestone (dark). Embayed windward 
reef edge and algal ridge at left curves around in distance, Onotoa Atoll, Gilbert Islands. Photograph by P. E. 
Cloud, Jr. Below: Flourishing coral growth at surface of leeward ocean reef without algal ridge, Bikini Atoll, 
Marshall Islands. Photograph byj. I. Tracey, Jr. 
