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PAPERS FROM THE DEPARTMENT OF MARINE BIOLOGY. 



the primaiy septa are well developed, but do not reach the center, the directives may 

 be somewhat more prominent than the others of the cycle; the secondaries much smaller, 

 but usually distinct. 



Coenenchyma compact, surface closely granulate, without definite costules. 



Specimen No. 2 {plate 80, figs. 3, ja, ji) : _ 



Corallum cespitose, corymbose, rising from an incrusting base. Height of colony about 

 14 cm., horizontal diameter about 25 cm. There is some distal anastomosis of the branch- 

 lets. The dimensions of branches and branchlets of this specimen are as follows: 



Measurements of branches and branchlets of Acropora variabilis. 



Axial corallites, dimensions ajid exsertness as stated in the table; subcylindrical in 

 form, summit flattish, with the upper edges of the walls slightly rounded, except on some 

 abnormal branches near the base in which there is slight tapering with constriction toward 

 the aperture. Walls reticular; texture loose in young, dense in old corallites; thickness 

 about the same as or slightly greater than the diameter of the aperture; outer surface 

 granulate, indistinctly costulate. Aperture broadly elliptical; diameter about or sHghtly 

 less than one-third that of the corallite. Septa, primaries well developed, subequal, or 

 the directives somewhat more prominent; smaller secondaries usually obvious. 



Radial corallites relatively uniform in size; appressed tubiform or ascending tubiforrn; 

 frequently arranged in somewhat irregular rows. Usual size 3.5 mm. long, 1.5 mm. in 

 diameter, with the calice of one corallite at the base of the one next higher. Tubiform 

 corallites may be 4.5 or 5 mm. long and 2 mm. in diameter. A few subimmersed or small 

 appressed tubular corallites are scattered between the bases of the larger corallites. 

 Toward the base of the branches most corallites become immersed or appressed nariform; 

 over considerable areas at the branch bases there may be no calicular apertures. In the 

 appressed corallites the outer wall is thicker than the inner and rises slightly higher, thus 

 causing the aperture to be inclined; often it curves just below its upper edge so as to be 

 nearly parallel to the branch axis; its texture is reticular, becoming compact in old corallites; 

 its outer surface granulate, and in places obscurely costulate. In free tubular corallites 

 the inner wall may almost equal the outer in thickness. The apertures are elliptical or 

 oval in outline, nariform in the appressed and elliptical in the free tubular corallites; the 

 diameter approximately one-third that of the corallite. The directive septa distinct, the 

 other primaries usually recognizable but smaller; very small rudimentary secondaries can 

 usually be distingui hed. 



Coenenchyma finely echinulate, flaky; becomes dense. 



A comparison of the descriptions of No. i and No. 2 shows that they are similar 

 as follows: (i) growth-form and general facias; (2) in the structural characters 



