CORALS FROM MURRAY, COCOS-KEELING, AND FANNING ISLANDS. 165 



tive septa are wide and often either meet or nearly meet deep down in the calice; the outer 

 directive is on an average more developed than the inner; the other primaries present, 

 but are short and rather inconspicuous; a complete cycle of inconspicuous, rudimentary 

 secondaries can usually be distinguished. On older parts of the corallum, two cycles are 

 clearly developed; the outer wall is thickened and curved toward the upper directive septum. 

 Immersed corallites, about 0.75 mm. in diameter, occur between the bases of the protu- 

 berant ones. Two cycles of septa, the second very small, can frequently be distinguished 

 in these. 



Coenenchyma flaky, granulate-costulate, with perforations between the costules. 



Station, Murray Island. — Southeast reef, line I, 1,400 feet from shore; depth 

 14 to 15 inches; bottom rocky, broken coral. 



Two specimens, both only fragments, have greatly perplexed me, but structur- 

 ally they are so similar to A. haimei that I am referring them to that species. Com- 

 parison of the figures and the descriptions show their similarity. The variant has 

 thicker branches, a more arborescent growth-form, and the radial corallites are 

 rarely or not at all compressed laterally. Near its base, however, the lower wall 

 of the radial corallites is thickened, and is curved toward the upper directive, 

 similar to the condition in A. haimei. 



Acropora (Eumadrepora) decipiens (Brook). 

 Plate 67, figures 2, 2a, lb, specimens from Murray Island. 

 1893. Madrepora decipiens Brook, Cat. Genus Madrepora, p. 51, plate 14, figs. B, C, D. 



The following is a description of a specimen of Acropora decipiens from Murray 

 Island: 



Corallum with an expanding base, 137 by 165 mm. in diameter, above which rise stout 

 stems. Lesser basal diameter of the stems between 25 and 30 mm., greater basal diameter 

 up to nearly 35 mm. The branches tend to spread laterally; some may bend downward 

 and fuse to the basal expansion, and there is considerable anastomosis, both with a tend- 

 ency toward plate formation at the bases and with cross-fusions nearer the periphery. 

 The horizontal diameters of the colony are 230 by 287 mm.; height about 90 mm. Diam- 

 eter of a branch from 12 to 17 mm. about 50 mm. from the end. The cross-section is sub- 

 circular or somewhat compressed radially. 



Axial corallites rounded, short conical, or protuberant with plane margins; diameter 

 from 3 to 4 mm. across the aperture; exsert, about 2 mm.; walls thick, reticular, porous 

 or considerably compacted, externally costulate; 2 well-developed cycles of septa, the pri- 

 maries nearly meeting, secondaries about half as long. The form of the axial corallite 

 appears to be largely determined by environmental condition. The specimen here de- 

 scribed is from the Lithothamnion ridge, where the water is relatively rough and perhaps 

 there is more or less detritus in suspension. Whatever may be the cause, the rounded and 

 short conical corallites are mostly on the upper surface, while the protuberant, subcylindrical 

 axial corallites with flattish margins are around the periphery. 



Radial corallites strikingly unequal in size; on the lower surface except near the periph- 

 ery immersed or subimmersed; diameter about 1 mm., about I mm. apart; as the branch 

 ends are approached, the lower lips of some corallites become more prominent, with resulting 

 short labellate, gutter-shaped, dimidiate, and nariform apertures, maximum length about 

 3.25 mm. On the upper surface are small immersed or subimmersed corallites; many small 

 corallites have a thin, spout-shaped lower wall, diameter 1 mm. or less. The larger corallites 

 range in height from 3 to 5 mm.; in diameter, from 1.5 by 2 mm. to 2.5 by 3 mm.; their 

 distance apart about I to 3 mm. On the sides of the larger corallites, especially near the 

 base, are small corallites with thin, produced lower walls and entirely without upper walls. 

 Except near the branch ends, where they are slightly ascending, the large corallites stand 

 perpendicular to the branch surface. The form ranges from dimidiate through nariform 

 to tubo-nariform or tubular with an oblique aperture. The lateral compression is consider- 

 able; in many calices the outline of the aperture is narrow elliptical. The outer wall is 

 rather thick, at first porous; later it becomes compact, but synapitculae may still be recog- 

 nized between the costules; it may slightly recurve or slightly incurve at the margin. The 



