50 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery X. of the floor, each lamina corresponding to a space between 

 two mesenteries ; then the lamina itself is built up and so 

 forces the skin inwards into the said space. Thus the 

 skeleton takes the shape of a cup or calyx, partly divided by 

 vertical partitions called septa. Other growths from the wall 

 or from the floor of the cup may be formed in like manner. 

 These are : a single spike or column in the centre, called the 

 " columella " ; columns between the ends of the septa and 

 the centre, called pali " ; ridges outside the wall, corre- 

 sponding with the septa inside, and called " costae." As the 

 calyx grows upwards, the polyp is sometimes pulled away 

 from the bottom of it, but does not therefore stop the 

 secretion from its skin; if a small piece only is pulled 

 away from one side, the skin builds here an oblique partition 

 called a " dissepiment ; " if the whole base is pulled away, 

 it deposits a horizontal or saucer-shaped, or sometimes 

 funnel-shaped, partition called a " tabula." Such is the 

 general structure of a solitary cup-coral. Corals may form 

 colonies, either by the repeated budding of such a single form 

 or by its dividing down the middle into two, each half again 

 dividing, and so on. This process of fission, as it is called, 

 is sometimes incomplete, and so arises a form like the Brain- 

 coral, in which the cavities of the polyps and of the cups 

 remain connected in serpentinous grooves (Fig. 24). There 

 is much the same difficulty in connecting Palaeozoic genera 

 with the Madreporaria of Tertiary and recent date, as we 

 have already seen attaching to other groups of Coelentera. 

 Similarly the modern reef-builders, Madreporidae and Pori- 

 tidae, first appear in Tertiary rocks. Consequently the 

 classification of the Order is far from settled. By means of 

 the skeleton it is possible to divide the genera into three 

 groups : Aporosa, Fungacea, and Perforata. The Aporosa 

 are so called because the calyx-wall and the septa are not 

 perforated by canals, and in colonial forms the polyps are 

 either separate or connected only by superficial canals. The 

 Fungacea include all forms like Fujigia, whose skeleton 

 has so many septa that it looks like the under side of a 

 mushroom ; the long thin septa of these forms are strutted 

 by short cross-bars. The name Perforata is given to corals 

 in which the skeletal substance is porous throughout, and 

 the polyps of a colony are connected by deep-seated canals. 

 It is not pretended that these groupings indicate relationship. 

 Another method of division is according to the arrangement 

 of the septa. In later Madreporaria these conform to the 



