MADREPORARIAN CORALS 59 



West Indies to the genus Eusmilia and specimens from the 

 East Indies and Pacific to the genus Euphvlha. 



The characters of the polyps in the three genera seem to 

 be very much ahke. In a Hving specimen there is an oral 

 disc surrounding the mouth, and at the margin of the calyx 

 there is a large number of short finger-shaped tentacles. 

 Outside the margin of the calyx the soft living substance 

 extends downwards for a few millimetres like a finger-stall 

 covering the hard corallum. This outer skin is called the 

 Edge-zone by Enghsh authors (in German " Randplatte "). 

 Below the Edge-zone the corallum is exposed, and is usually 

 subject to the attacks of boring worms and other destructive 

 agents, or is partly protected by Polytrema or Polyzoa or 

 other encrusting forms of animal and vegetable life. 



The " Edge-zone " has another point of interest, as its 

 lower limit can be fixed in the coral after the removal of 

 the soft parts by the texture of the surface. Above the 

 limit the surface is compact and marked b}^ more or less 

 well-pronounced costal ridges ; below the limit the surface 

 is chalky in texture, and there is no trace of costal 

 ridges. 



i\ccording to Bourne, " the Mussa of Diego Garcia is of 

 a dull brown colour, with olive-green disc and tentacles." 

 According to Ehrenberg, the polyps are pale brown with 

 a golden-yellow disc. Duerden describes the colours of 

 another genus — Isophyllia, closely related to Mussa — found 

 on the reefs of Jamaica as follows : " The prevailing colours 

 are dark green, brown, and yellow, with minute, opaque 

 white, superficial granules distributed practically all over. 

 The yellow colour predominates along the thecal ridges and 

 the green along the valleys. Irregular, opaque white, cream, 

 or green patches are sometimes present on the disk, ending 

 in streaks towards the periphery, that is, in the region 

 covered by the overfolding column wall." 



The Astraeidae that have so far been described have the 

 more characteristic massive, spherical, or lobed form of the 

 members of this family. Some statement must now be 

 made concerning the corals that clearly belong to the family 

 but have a different appearance. They may be arranged in 



