618 DYNAMICAL GEOLOGY. 



in the distribution of vegetation over the land. Some of the kinds 

 branch like trees of small size or shrubs [Madreporce) ; others form 

 closely-branched tufts (Pocilloporce, many Pontes) ; others resemble 

 clustered leaves [Merulince, Manoporce), or tufts of pinks ^Tubiporcc)., 

 or lichens and fungi (Agaricice, etc.) ; others grow in hemispherical 

 or subglobular forms ( Astrece, Meandrince, and some Pontes) ; and 

 others are groups of slender, brilliantly-colored twigs (Gorgonicc). 



When alive in the water, all these corals are covered throughout 

 with expanded polyps, emulating in beauty of form and colors the 

 flowers of the land. 



The most common groups of reef-forming corals are the Madrcpora, Pocillo- 

 pora, Porites, Astrea, Meandrina, and Milhpora. 



1. JIadrepora. — Corals usually neatly branched ; branches with pointed ex- 

 tremities, each ending in a small cell or calicle ; surface covered with calicles (or 

 prominent polyp-cells) about a line in diameter. 



2. Pocillopora. — Corals closely branched, with uniform width of interval 

 between: branches blunt at the extremity; surface covered with angular 

 prominences, a line or two thick, each containing several polyp-cells ; spaces 

 between the prominences also covered crowdedly with polyp-cells ; texture of 

 the coral in its interior mostly solid. 



3. Porites. — Corals often branched; the branches blunt; surface nearly smooth, 

 covered throughout with polyp-cells less than a line in diameter; some of the 

 species massive, irregularly globular, and occasionally ten or fifteen feet in 

 diameter ; texture of the coral very finely cellular. 



4. Astrea. — Corals massive, usually hemispherical, covered with radiated 

 polyp-cells, often half an inch or more in diameter; the hemispheres sometimes 

 fifteen or twenty feet in diameter. 



5. Meaitdrina. — Corals as in the Astrea group in form and size, but surface 

 covered with meandering furrows, often a quarter of an inch or more in width. 

 Often called brain-coral, in allusion to the meanderings in the surface of the 

 brain. 



6. Millepora. — Corals branching, lamellar, massive; surface smooth; cells 

 exceedingly minute, and in the interior of the coral divided by horizontal par- 

 titions (a characteristic called tabulate by Edwards). 



Each of the polyp-cells in these corals corresponds to a separate animal or 

 polyp (p. 163). In the Madrepores, the polyps when expanded have twelve rays 

 or tentacles, with a diameter of an eighth to a quarter of an inch. Those 

 of the Pocillopores and Porites are also twelve-rayed, but smaller. The Astrcas 

 have an indefinite number of rays or tentacles: in some species of the family 

 the expanded flower-like polyp is an inch or more in diameter. In the Mean- 

 drinas the polyps coalesce in lines; there is a series of mouths along the centre 

 of each furrow, and a border of tentacles either side. 



In the Millepores, as stated on page 162, the animals are Acalephs, and not 

 true polyps. 



Another common group of corals is the Fungia: they have the form of broad 

 circular or oblong disks. In many of the species the disk corresponds to a 

 single polyp, and has a diameter in some cases of ten or twelve inches. 



