ANTHOZOA ZOANTHARIA. 743 



fathoms. With one or two exceptions the deep sea forms are widely 

 distributed and most of them occur also in shallow water. Leptopenus, 

 however, is an instance of a coral limited to the deep sea (1500-2250 

 fathoms). The majority form the well-known coral reefs. These struc- 

 tures, which are absent from the west coasts of America and Africa, are 

 for the most part confined within the limits of 30 North and South of the 

 equator, where the mean temperature of the sea does not fall below 68F., 

 growing in water of, as a rule, not greater depth than twenty fathoms and 

 for the most part less, different genera and species flourishing at different 

 depths. The coral banks may follow the coast line of the land either 

 closely as ' fringing reefs ' ; at a distance, sometimes great, as ' barrier reefs '; 

 or they inclose spaces of water, the * lagoons/ and then constitute ' atolls/ 

 They are formed in the first instance on sub-marine banks and elevations 

 mostly of volcanic origin, combined with deposits of mud and calcareous 

 matter ; and their subsequent extension depends on various factors, * the 

 temperature, solvent power, currents, tides, and waves of the sea ; ' on ' the 

 amount and direction of the supply of pelagic food, the up-building of cal- 

 careous deposits to the zone of reef-builders, the outward vigorous growth 

 of the coral-masses, and their death and decay ' forming a fresh extension 

 of ground ' and the solution of their skeletons in the inner parts of the 

 reefs ' (Geikie). Neither subsidence nor upheaval of the land are necessary 

 factors, though both may occur, especially the latter. There are large 

 numbers of fossil Madreporaria known. The existing fauna appears to be 

 but sparingly represented in Palaeozoic times during which flourished a 

 series of extinct and peculiar forms (infra). Some of the existing groups 

 were, however, formerly more numerously represented than at present, e. g. 

 Turbinolidae in the Chalk and Eocene. 



The sub-class Zoantharia is divisible into the Actiniaria, Antipatharia, and 

 Madreporaria, the characters of which, as well as of the tribes of Actiniaria, have 

 been already given. 



The classification of the Madreporaria presents great difficulties. The anatomy 

 of their soft parts has been little investigated, and the characters of the skeleton 

 alone are not sufficient basis for any sound classification. Moreover, these characters 

 are not trustworthy in all respects ; see p. 739. A principal division into Perforata 

 with a skeleton or coenenchyma, perforated throughout by cavities which lodge, so 

 far as is known, coenosarcal tubes, and Aporosa in which the corallum is not so 

 perforated, has been universally recognized. The most recent re-arrangement of the 

 sub-groups of these two main divisions is by Martin Duncan, J. L. S. xviii. 1885. 



The Palaeozoic Corals are for the most part classified as Rugosa s. Tetracoralla, 

 but the assemblage is probably artificial. The corallum is simple or colonial, but 

 there is no coenenchyma ; it is free or fixed. The septa are arranged in four systems, 

 which are either disposed in a bilaterally symmetrical manner and for the most part 

 feathering from a primary chief septum and two lateral septa, or else are regularly 



