MADREPORARIAN CORALS 95 



however, and the corals are exposed for a time to the sun, 

 the briUiancy to a great extent disappears, and a uniform 

 duhness of brown and yellow seems to prevail. 



PoRiTES. — The genus Porites is another very important 

 reef-building coral widely distributed in the tropical seas of 

 the Old and New World. In some seas, blocks of Porites 

 reach to an enormous size,^ and appear to be the principal 

 factors in the construction of the reefs, but in others the 

 Porites rocks occupy a subordinate position to the colonies 

 of Madrepora, and relatively small blocks of it are scattered 

 about in more or less isolated positions. 



The forms assumed by the large colonies are almost as 

 varied in Porites as they are in Madrepora ; but in Porites 

 massive, spherical, lobate, and encrusting forms are more 

 characteristic. Ramified forms are found, but the branches 

 are generally thick and terminate in blunt knobs. The 

 ramification is seldom profuse. 



The surface of the corallum is seen to consist of a very 

 large number of small calices with common pentagonal 

 thecal walls (Fig. 39). There is no coenosteum between the 

 calices. The septa are twelve in number and the directive 

 septa are not usually distinguished from the others by their 

 greater size. All the septa are so profusely perforated that 

 each one has the appearance of a lattice work of trabeculae 

 rather than that of a perforated lamina. On the free border 

 of the septa there is a row of blunt spines, and on their 

 inner side there is a cycle of pali. In the centre of the calyx 

 a single spine represents a columella. At the base of the 

 calyx the septa are often seen to be connected by syn- 

 apticula. The substance of the corallum below the surface 

 has usually the appearance of a most intricate maze of 

 trabeculae, but frequently the cavities become filled with 

 stereoplasm and, less frequently, the trabeculae show a 

 tabular arrangement. 



The living polyps of Porites project but slightly from 

 the calices. The tentacles are usually twelve in number, 

 digitiform or acute in specimens in Jamaica (Duerden), or 



^ For example, the great mass of Porites, 30 to 40 feet in diameter, 

 described and photographed by Saville-Kent. 



