26 MADREPORARIA. 
regard all as varieties of one species ; and there appears little reason to doubt that the 
ultimate form of the corallum, whether flabellate or arborescent, is to a great extent 
dependent on the environment. Tracing the frondose and vasiform specimens, without 
branchlets, back to typical M. prolifera, the first step is seen in those specimens already 
referred to in which the course of the branches composing the fronds is indicated by 
grooves in the surface. In other specimens the branches, though confluent, are well- 
marked at the apex of the fronds, and an increase in the size of the axial corallites is 
generally observable in such specimens. Next in the series come two ‘ Challenger ” specimens 
from St. Thomas, which show the characters of M. prolifera so unmistakably that they 
were referred to that species by Quelch. In one of these the branches form a complanate 
but not a solid frond; usually from 2 to 5 branchlets are collected together in flattened 
groups and fused together laterally ; they vary from 7 to 12 mm. in thickness, including 
the radial corallites. Axial corallites 2 to 8 mm. in diameter, with thick and very porous 
wall. Radial corallites spreading at an angle of about 45°, tubular, with an oblique 
aperture, or nariform, rather unequal, 1 to 3°5 mm. long, with immersed ones between, 
especially in the lines of fusion. The other specimen has stouter and less confluent 
branches, with radial corallites which, in some parts, recall the condition characteristic of 
M. cervicornis. 
B. Forma prolifera. 
Corallum consisting of much-divided branches which radiate obliquely from a common 
centre ; a typical specimen in the collection of the British Museum is 18 em. high and 
38 em. broad. Main branches about 20 em. long and 1‘7 cm. thick at the base; sub- 
divisions numerous, chiefly lateral and ascending, the narrow angle between the divisions 
frequently filled up by sclerenchyma for some distance ; middle divisions about 1 em. thick, 
distal branchlets 2 to 6 em. long, 6 to 10 mm. thick, including the corallites ; often a dozen 
or more branchlets form a fan-shaped proliferous group at the distal extremity, 8 em. long 
and 9 cm. wide. Axial corallites very variable in size in different parts of the specimen, 
sometimes 2 mm. diameter, relatively thin-walled, and only 2 mm. exsert, often with oblique 
aperture, and scarcely differing from the lateral corallites; others have a maximum diameter 
of 3°5 mm., and are 3 to 5 mm. exsert, sometimes apparently more owing to the extremely 
small size of the buds on the lower part. adial corallites usually rather crowded, tubular 
with oblique aperture, tubo-nariform or nariform, often slightly compressed on the distal 
parts, usually 4mm, long and 1°5 mm. diameter, but becoming shorter and stouter, with 
pore-like aperture, on the basal parts. The under surface of the branches has more distant 
and irregular corallites, often dilated and appressed. Wall of the corallites on the upper 
surface rather thin and striato-reticulate at first, becoming thicker and more distinctly 
echinulate with age. Star usually not prominent, the directive septa are moderately 
developed, but the remaining members of the primary cycle are more or less rudimentary. 
The specimen referred by Ehrenberg to M. pocillifera, Lamarck, which M.-Edwards 
supposed synonymous with his M. ehrenbergi, but which is quite distinct from that species, 
