MADREPORARIAX CORALS 85 



A few words must now be added on the difference be- 

 tween the two genera. 



Seriatopora. — vSeriatopora can usually be distinguished 

 at once from Pocillopora by its slender and sharpl}^ pointed 

 terminal branches (Fig. 34) . The calices are arranged in longi- 

 tudinal rows on all sides of the branches and in some species 

 show a margin raised above the level of the coenosteum. The 

 two directive septa form a prominent ridge on the floor of 

 the cup, and this ridge is always parallel with the axis of the 

 branch. It may be a matter of dispute whether the middle 

 part of this ridge should be called the columella, but the 

 most reasonable point of view seems to be that there is no 

 columella. The other septa are often very rudimentary 

 and difficult to see. In some specimens there are only four 

 and in others eight, or if we count the two directive septa, 

 six or twelve in all. 



The colour of living Seriatopora on the reefs is usually 

 pink, but yellow varieties have been found. In some cases 

 the polyps appear as brown spots on the branches. 



Pocillopora. — In Pocillopora the method of branching 

 is coarser and more irregular than in Seriatopora, and the 

 terminal branches are thick and blunt at the apex, never 

 being drawn out into fine points. The surface of the branches 

 is very rough and in many species raised into a series of 

 little mounds or verrucae. The calices are very numerous, 

 and as compared with Seriatopora v'ery close together, so 

 that in many places they are actually in contact with one 

 another. The study of the septa of these minute calices is 

 beset with even greater difficulties than in Seriatopora, 

 because in many specimens their cavities are filled up with 

 a chalky deposit (stereoplasm), which completely hides the 

 structures buried in it and cannot be removed by the 

 ordinary cleaning reagents. However, when a terminal 

 branch of a good specimen is examined in a strong light with 

 a lens, a ridge formed by the directive septa and parallel 

 with the axis of the branch can usually be made out. A 

 more detailed examination of this ridge with a higher power, 

 however, shows that in the middle of it there is usually a 

 definite but small papilliform columella. In addition to the 



