MADREPORARIAN CORALS 47 



rapidly growing genera of the younger families — the Madre- 

 poridae and Poritidae — of the Perforate corals. 



Most of the Astraeidae are colonial corals, and give rise 

 by fission or gemmation to large heavy rocks of limestone, 

 usually spherical or lobed in form, and rarely, among recent 

 genera, dendritic in growth, and, if dendritic, never dividing 

 up into pointed terminal branches. 



The calices are usually set close together, and in many 

 genera are actually in contact with one another, so that there 

 is little or no coenosteum between them. The septa are 

 numerous, and may be entire and smooth as in the " Astrsees 

 inermes " of Milne-Edwards and Haime, or dentate, spined, 

 or ragged as in the " Astrsecs amies " of the same authors. 

 But the septa nev^er meet and fuse together as they approach 

 the centre of the calyx as in some of the Eupsammiidae (see 

 p. 75), nor are they connected together by synapticula as 

 in the Fungiidae. 



One of the most important features of the family is that 

 as the cah'x increases in length by upward growth at the 

 surface, the lower part of the cavity of the calyx becomes 

 shut off from the upper by calcareous structures, for which 

 w^e may use the general term " Endotheca." The poh'ps 

 and other tissues of the coral are entirely confined to the 

 surface and to the cavities of the calices down to the level 

 of the endotheca. It is the presence of endotheca which is 

 the only character to distinguish some genera of Astraeidae 

 from genera of Oculinidae and of Turbinoliidae, which in 

 other respects are very similar to them. 



There is unfortunately some confusion in the use of 

 the technical terms that are employed for the different 

 kinds of endotheca, and it is difficult to give any terms 

 a very precise definition owing to the great variety of 

 form that the endotheca assumes, but the diagrams in 

 Fig. 15 will show the three principal varieties that are 

 found. 



When the endotheca is in the form of transverse plates, 

 the plates are called Tabulae ; when it is in the form of 

 irregular laminae, the laminae are called Dissepiments ; 

 when it is more or less of granular consistency, filling up the 



