Rathbun.] : 40 {May 15, 
the reef and the shore there is an average depth of water of about six 
or seven feet only. The upper and outer portions of the reef are 
very irregular, but the inner part is comparatively smooth. No large 
corals are living upon it; at the northern end were collected a few 
small specimens of Porites, and toward the south a few millepores, 
Symphyllia (?), and Porites. Much of the bottom surrounding 
the reef is very rich in coral growth. No sections were obtained 
giving us a clue to its structure, which is, however, probably the 
same as that of the Itaparica reef. 
In my former description of the Itaparica reef I stated that, while 
the lower portion was plainly made up in large part of true corals, 
the upper part appeared to contain only nullipores. I have since 
found that the worm tubes covering the surface of the reef enter 
very largely into its structure, probably to as great an extent as the 
nullipores, and give rise to an exceedingly hard, calcareous rock 
from which, ultimately, all traces of the worm-tube structure disap- 
pear. The worm tubes and nullipores evidently compose the entire 
upper half of the reef. The nulliperes, in the upper portions, so far 
as my observations went, were all of the encrusting lichen kind, and 
resulted in a compact structure, showing a sort of wavy lamination 
which is due to the successive growths of nullipores. The large dig- 
itate nullipores, so common at Pernambuco and at many places in the 
bay of Bahia, are limited to the lower part of the reef, where they 
are associated with the true corals. At present nullipores are living 
in abundance only on the outer side of the reef, to a height of about 
one foot above medium low tide. Above the line of nullipores we 
find the entire upper surface of the reef coated with a layer of living 
worm tubes and large barnacles. The latter are generally broken off 
by the waves when dead, but the former remain, producing a loose 
structure near the surface, which becomes more compact below. 
The existence of nullipores in this upper portion indicates, however, 
that they lived on top of the reef at no distant time, and probably 
also that the reef has been elevated to a slight extent since then. 
Within the reef the water is very shallow, being deepest near the 
reef and especially at and around the openings through it; it gradu- 
ally shallows inwards toward the beach. The bottom of this shallow 
inner channel is covered with sand and fragments of all sizes of cor- 
als and shells. Corals were not found in an upright position in this 
channel, nor do living corals exist there at all. The coral fragments 
are all old, frequently much worn, and almost invariably covered 
