224 A. JE. Yerrill — Corals of the Genus Acro2yora. 



is from Macclesfield Bank. To me it seems much like M. alliomor- 

 p/ia Brook. 



The true appressa Ehr. has been redescribed by Brook from the 

 type in the Berlin Museum. It has not been figured. It seems to 

 agree pretty closely with Dana's species in most respects, but has 

 larger axial and radial corallites than I have observed in the latter. 

 According to Brook the larger radial ones are up to S'"'" long and 

 2mm thick; the axial 2-2. S™'" in diameter. lean find no other marked 

 difference, and possibly this is not beyond the limits of variation in 

 this species, for many others are known to vary more than this in 

 the size of the corallites. The upright branches are shorter than in 

 Dana's type (one inch long, t. Ehr.; 4-6'=", t. Brook). 



Brook also states that the wall is " dense and echinulate, not 

 striate." I have shown above that this character is variable in 

 Dana's form. According to Brook the radial corallites are mostly 

 "hooked labellate, with a thick, blunt, and frequently incurved 

 apex." The larger size of the axial corallites (t. Ehr.) formerly led 

 me to suppose that it might be identical with M. acervata Dana, but 

 the measurements given by Brook would seem to contradict this, 

 unless the type be undeveloped in this respect ; but Ehrenberg states 

 that it is nine inches in diameter. 



The distinctions between assimilis and alliomorpha seem to me 

 very slight and at most only varietal. See also A. cUssimilis, below. 



Acropora arbuscula (Dana) Ver. See p. 2l3. 



Maclrepora arbuscula Dana, Zooph., p. 474, pi. xl, fig. 2, 1846. Brook, op. 



cit., p. 40. 

 ? M. laxa Lam., Brook, p. 46. 



This species, in a large series from Singapore, shows much varia- 

 tion. There are two branches of Dana's type from the Sulu Sea, in 

 the Yale Museum (Nos. 2005, 4165) besides a good series from 

 Singapore. 



The type differs a little from most of the latter in having the 

 radial corallites rather shorter and more squarrose, with the walls 

 more porous and more distinctly costulate. But these characters 

 vary in this species. 



In the type the radial corallites of the upper side of the branch are 

 rather crowded, subequal, short (about 2-3'"'^ long and 2°"'" broad), 

 tubular, with lower wall a little thicker, obliquely truncate, and with 

 the aperture terminal and a little oblique, owing to the thinner and 

 shorter inner lip. The outer lip is distinctly thickened, rounded, but 



