26 MADEEPORAEIA. 



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 3 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 cm. high and 

 38 cm. broad. Main branches about 20 cm. 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 cm. thick, 

 distal branchlets 2 to 6 cm. 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 cm. long 

 and 9 cm. wide. Axial corallites very variable in size in difl'erent 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. Radial corallites usually rather crowded, tubular 

 with oblique aperture, tubo-nariform or nariform, often slightly compressed on the distal 

 parts, usually 4 mm. 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. 



