Rathbun.] 40 [May 15, 



the reef and the shore there is an average ileptli of water of about six 

 or seven feet only. The upper and outer portions of the leef 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 towards 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 descrij)tion of the Itaparica reef I stated that, while 

 the lower portion was plainly made up in large part of true corals, 

 the upper part appeai-ed to contain only nullipores. I liave 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 nullipores, in the upper ])ortions, 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, ai"e limited to the lower part of the reef, where tliey 

 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 tulies 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, liowever, 

 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 tlie 

 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 



