70 MADEEPOEAEIA. 



many are short and verruciform ; all the corallites on the under surface of the corallum are 

 immersed. Upper surface provided with more numerous suberect branchlets, which usually 

 arise singly; length 1'3 to 2 cm., occasionally more, diameter 4 to 5 mm. at the base; a few 

 are forked, and others near the margin are more important and bear radiating branchlets 

 of a second order. Axial corallites 2 mm. diameter, 1"5 mm. exsert; wall usually very 

 thin ; aperture large and round or oval with 6 narrow subequal septa, lladial corallites 

 immersed or subimmersed, excepting near the apex of the branchlets, where a few are short, 

 round nariform ; the marginal branchlets usually bear more or less prominent corallites to the 

 base, 1 mm. diameter, and the outer part of the wall rarely over 1-5 mm. long; wall thin, 

 aperture wide and circular ; star scarcely developed, but sometimes the directive septa may be 

 recognized. Corallum very porous and fragile ; surface striate and strongly spinose ; wall 

 striate and echinulate. 



Indian Ocean : Mauritius. 



a. Mauritius. Purchased. 92. 1. 3. 4. (Type.) 



54. Madrepora amhigua. (Plate VIII. fig. C.) 



Madrepora ambigria. Brook, Ann. Mag. N. H. 1892, vol. x. p. 451. 



Corallum subhorizontal or erect, somewhat flabellate ; branches irregularly confluent ; 

 height 24 cm., diameter 36 cm. Base incrusting. Basal parts of the branches fused into a 

 solid mass, with mammiform protuberances. Branches 1'5 cm. diameter, bearing on the 

 upper surface a few short arched and blunt divisions scarcely smaller in diameter, but those 

 near the margin may be longer and not quite so thick ; length 2 to 4 cm. Axial corallites 

 scarcely prominent unless near the margin of the colony, 2"5 to 3 (rarely 3"5) mm. diameter. 

 Radial corallites very unequal, many are immersed. Prominent corallites irregular in arrange- 

 ment, chiefly spout-shaped, spreading, unless near the apex of a twig ; inner part of the wall 

 not prominent, except in the stouter corallites ; outer part thin when short, but thick and 

 keeled, with a rounded margin, in the stouter corallites; length 0'5 to 2*5 mm., diameter 

 1 to 2 mm. The star consists of 6 moderately developed septa ; sometimes the lower directive 

 is very prominent and the others very narrow. Wall strongly striate and dentate or echinu- 

 late, surface reticulate and echinulate. On the under surface of the colony the branches are 

 not flattened, but the interval between them may be filled up with coenenchyma ; there are 

 no branchlets springing from the under surface, where the corallites are numerous and all 

 immersed. 



Pacific Ocean : Great-Barrier Reef area. 



a. Northumberland Island. Saville-Kent Coll. 92. 6. 8. 278. (Type.) 



55. Madrepora complanata. (Plate IV. fig. C.) 

 Madrepora complanata, Brook, Ann. Mag. N. H. 1891, vol. viii. p. 459. 



Corallum flabellate, probably about 23 cm. high and 40 cm. broad. The main stem gives 



