70 MADREPORARIA,. 
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 em., 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, 15 mm. exsert; wall usually very 
thin; aperture large and round or oval with 6 narrow subequal septa. Radial 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 15 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 ambigua. (Plate VIII. fig. C.) 
Madrepora ambigua, Brook, Ann. Mag. N. H. 1892, vol. x. p. 451. 
Corallum subhorizontal or erect, somewhat flabellate; branches irregularly confluent ; 
height 24 cm., diameter 836 em. 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 2to 4cm. Axial corallites 
scarcely prominent unless near the margin of the colony, 2°5 to 3 (rarely 35) 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, cxcept 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 
lto2mm. 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 ceenenchyma; 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 
