50 GUIDE TO THE FOSSIL INVEETEBKAT]’: ANIMALS. 
Gallery X. 
of the Hour, each lamina correspomlin^f to a space between 
two mesenteries ; then the lamina itself is bnilt np and so 
forces the skin inwards into the said space. Tlius the 
skeleton takes the shajje of a cnp or calyx, partly divided by 
vertical ])aiTitions called se])ta. Other growths from the wail 
or Irom the iioor of the cup may be formed in like manner. 
These are : a single spike or column in tlie centre, called tlie 
“ columella ” ; columns between the ends of the .septa and 
the centre, called “ixili”; 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 tlie whole base is pulled away, 
it deposits a liorizontal or saucer-shaped, or sometimes 
funnel-shaped, partition called a “ tabula.” Sucli is the 
general structure of a solitary cup-coral. Corals may form 
colonies, either by the repeated bndding 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 (Tig. .‘14). There 
is much the same dilliculty in connecting i’alaeozoic genera 
with the ^ladreporaria of Tertiary and recent date, as we 
have already seen attaching to other giuups of Coelentera. 
Similarly the modern reef-builders, IMadreporidae 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 ])Ossible to divide the genera into three 
groups : Ayiorosa, Fungacea, and Perforata. The Aporosa 
are so called because the calyx-wall and the .septa are not 
perforated by canals, and in colonial forjus the polyps are 
either separate or connected only by superficial canals. The 
Fungacea include all forms like Fiou/ia, whose skeleton 
has so many septa tliat it looks like tlie under side of a 
mushroom ; the long thin septa of tiiese 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 
