240 A. E. Verrill — Corals of the Genus Acropora. 



The main branches spread out radially from the stout, one-sided 

 or subcentral base, and branch dichotomously ; the outer branches 

 lie nearl}^ in one plane, but usually do not coalesce to any consider- 

 able extent. 



The under side is covered with short irregular branchlets, directed 

 outwardl}'^, and somewhat appressed, each having one or several long 

 divergent, often crooked and tapered tubular axial corallites, up to 

 10 to 15"^™ long, and 2 to 2.5'"™ in diameter. Many of these tubular 

 corallites stand separately, or in clusters of 2 or 3, without any radial 

 corallites ujDon them ; but most bear at least a few small, appressed 

 radial corallites, and many terminate the small, irregular branchlets. 



The immersed corallites of the under side are scattered, not very 

 small, and have a conspicuous star. 



The coenenchyma beneath is very firm and dense, translucent, and 

 with a strongly vermiculated, rough surface, in many places sharjily 

 granulated. 



The ascending branches and branchlets of the upper side are much 

 subdivided dichotomously, the divisions forming acute angles ; those 

 toward the margins are often very proliferous. The smaller simple 

 branches are from 5 to 8™™ in diameter, and up to 20-25'"™ long ; 

 usually evenly tapered and acute. 



The axial corallites are slender (1.7 to 2'""^ thick), cylindrical, 

 and usually considerably exsert (up to 4-6""", rarely lO'"'") ; the wall 

 is moderately thick and very strong, though perforate, not swollen 

 at the margin ; its exterior is covered with regular and rather thin 

 costulae, between which there are rows of pores, in the grooves. 



The radial corallites are rather large, especially on the larger 

 branches, where they are nearly immersed ; the distal radial calicles 

 are about as large as the axial, or even larger. The distal radial 

 corallites are short and rather openly tubular, with the summit very 

 obliquely truncate, so that the inner lip is abortive, or nearly so ; the 

 outer lip is large, thin, often fenestrate, frequently a little narrowed 

 and incurved at the tip, but often flaring somewhat. These coral- 

 lites stand at angles of 30 to 45°, and are not appressed ; they are 

 about 2.'75 to S""" long, and about 2.5'^"" broad. The wall is thin, 

 porous distally, and covered with regular, sharply defined costae. 



The calicles are all distinctly stellate ; the radial ones have six 

 strong primary septa, the dir-ectives widest, and usually six very nar- 

 row secondary ones. The axial corallites have six wide and subequal 

 septa, usually with six very narrow ones of the second cycle. 



On the proximal part of the branches some of the corallites are 



