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PACIFIC SCIENCE, VoL XI, July, 1957 
The Reef Front is the shoreward face of a 
seaward reef, extending from the Reef Edge to 
depths of =±= 60 feet, the approximate dwindle 
point of flourishing coral growth. This habi- 
tat is ordinarily one of abundant living coral 
and other reef-building organisms. The shore- 
ward part of the Reef Front is characterized by 
turbulent water, particularly on a windward 
reef. 
The Outer Slope is the steeply descending 
seaward face of a seaward reef, extending from 
± 60 feet (the depth of marked dwindling 
of abundant living coral and coralline algae) 
to the deepest seaward portion of the reef. 
It is a habitat of few or no reef-building 
organisms, except at the transitional depth of 
approximately 60 feet. These deeper zones, 
the Reef Front and Reef Slope, are seldom men- 
tioned in this report, since, because of col- 
lecting methods, most of the gastropods 
treated here live intertidally or in relatively 
shallow water. 
A Lagoon Reef Flat borders the lagoon side 
of an island. It includes the intertidal zone 
and depths of a few feet below low tide line. 
It is generally a poorly developed, narrow 
reef, characterized by quiet water and sparse 
coral growth — quite a different environment 
from a Seaward Reef Flat. However, windward 
Lagoon Reef Flats of broad atolls, such as 
Bikini and Eniwetok, commonly exhibit fea- 
tures similar to those of a Seaward Reef Flat, 
because the long fetch of these lagoons allows 
the development of surf along the windward 
shore. Consequently species which typically 
inhabit Seaward Reef Flats are occasionally 
found living on these well-developed Lagoon 
Reef Flats (e.g., Drupa morum , Drupa ricinus). 
Quite often there is no reef on the lagoon 
shore. Instead there is a Lagoon Shelf. A Lagoon 
Shelf is a gently sloping lagoon shore, includ- 
ing the intertidal zone and depths of a few 
feet, and sometimes extending as deep as 20 
feet into the lagoon. The Lagoon Shelf is 
predominately covered with sand or detrital 
matter rather than by reef-building organ- 
isms. Often growing on this sandy shelf, and 
offering protection to animals, are seaweeds 
or small reefs. Gastropods which live on the 
Lagoon Shelf commonly burrow into the sand 
among eelgrass roots or at the base of small 
reefs (e.g., Terehra spp.). 
A Lagoon Reef Edge (Tracey, et al. Lagoon 
Reef Margin) is the lagoonward margin of a 
Lagoon Reef Flat. In some places the Lagoon 
Reef Edge is entirely absent. In others, it is 
ill-defined. In still others, such as the broad 
atolls of Bikini and Eniwetok, it is well 
developed and exhibits characteristics com- 
parable to a Seaward Reef Edge. 
A Lagoon Slope is a steeply sloping lagoon 
border extending from the lower or lagoon- 
ward edge of the Lagoon Shelf os Lagoon Reef 
Flat to the Lagoon Floor . The Lagoon Floor is 
the more or less level bottom of a lagoon. 
Subsidiary reefs and minor organic promi- 
nences which grow up from lagoon shelves, 
slopes, and floors are variously shaped as 
mounds, platforms, knolls, or pinnacles. 
Many are flat- topped. Names that have been 
used for such features are patch reefs, reef 
knolls, and coral knolls. In the present report, 
because of divergence in collecting notes, 
these structures are all referred to simply as 
Small Lagoon Reefs. 
Zoogeographic Distribution 
An outline of the geographic occurrence 
of each species is given. Such distributional 
patterns have never previously been assem- 
bled for the Indo-Pacific mollusks. Perhaps 
they will provide a point of departure for 
future work in zoogeography. 
Plate 2. Protected seaward reef margin and reef front pinnacle, Ine, Arno Atoll, Marshall Islands. Photographs : 
by P. E. Cloud, Jr. Above: Close view at low tide of irregualr edge of seaward reef margin, showing abrupt drop 
to depths at right. Brown, rose, and golden Pocillopora elegans (Dana) and gold, tan, and pink Acropora humilis 
(Dana) dominate the scene. Below: Underwater view of small pinnacle along and joined to the reef front a few i 
feet below the surface. Pocillopora , Acropora , and (at the top) the hydrozoan Milleporatenera Boschma are prominent. 
