96 CCELENTERATA. 



never form massive aggregations or " reefs;" This is due to 

 the fact that, when composite, the separate corallites are not 

 united together by a lax cellular coenenchyma, so that the 

 colony cannot increase to an indefinitely large size. The 

 deep-sea corals seem to have existed in all the great geological 

 periods, from the Silurian upwards. The chief genera of this 

 group at the present day are Caryophyllia, Balanophyllia, 

 Flabcllum, Desmophyllum, and Sphenotrochus, amongst the sim- 

 ple forms ; and Lophohelia, Amphihdia, Dendrophyllia, and 

 Astrangia, amongst the compound forms. 



The reef-building corals, when simple, are provided with 

 special structures which enable the polypes to grow rapidly. 

 The great majority of the reef-builders, however, are com- 

 pound, and owe the large size to which they attain to the 

 fact that the corallites are mostly united by a loose cellular 

 coenenchyma. The chief genera of reef-building Zoantharia 

 in Mesozoic, Kainozoic, and Recent times, are M&andrina, 

 Madrepora, Forties, Astraa, Millepora, Hcliopora, Cycloseris, 

 Trochoseris, Heliastrcea, Soleiiastraa, Pachyseris, Turbinaria, 

 and Astraopora ; but many others might be mentioned. 



Amongst the more ancient examples of coral-reefs may be 

 mentioned the Wenlock Limestone of the Upper Silurians in 

 England, some of the Devonian Limestones in North America, 

 and parts of the Carboniferous Limestone in various parts of 

 the world. In Mesozoic times coral-reefs existed towards the 

 close of the Trias in Western Europe, and largely in Oolitic 

 times both in Western Europe and in England. In the earlier 

 portion of the Tertiary period, again, vast coral-reefs were 

 formed in Central and Southern Europe, in Egypt, Syria, and 

 Arabia, and in parts of India. 



DIVISIONS AND DISTRIBUTION IN TIME OF THE ZOANTHARIA 

 SCLERODERMATA. The Zoantharia sclerodermata are divided 

 into the four following groups, founded upon the characters of 

 the corallum : 



1. Tabulata. Septa rudimentary, or entirely absent ; tabulae 

 well developed and dividing the space included within the 

 theca (the " visceral chamber ") into a number of stories 

 (fig. 46). 



2. Perforata. Septa well developed ; dissepiments rudi- 

 mentary ; no tabulae ; calcareous tissue of the coral (" scler- 

 enchyma ") porous. 



3. Aporosa. Septa well developed, lamellar ; no tabulse ; 

 calcareous tissue of the corallum (" sclerenchyma ") compact 

 and imperforate. 



4. Tubulosa. Septa indicated by mere strias ; thecae pyri- 

 form, occasionally united by a basal ccenenchyma (fig. 94). 



