v1 BRITISH FOSSIL CORALS. 
septa, the pali, and the columella, is always closed at its bottom and open at its upper 
extremity, where it usually presents the appearance of a sort of radiated cup, and constitutes 
the calice. In some species, this central cavity, or vesceral chamber, remains com- 
pletely pervious from one extremity of the corallum to the other; and the membranous 
appendices containing the reproductive organs, and situated im the loculi, extend to its 
basis, without encountering any obstacle ; but in other species a certain number of trans- 
verse ¢trabicule or synapticule extend from one septum to another at various heights, and 
fill up, more or less completely, the inferior part of the loculi. In other cases, horizontal 
or oblique laminze occupy the same position, and subdivide the loculi into a series of small, 
regular cells ; and sometimes these partitions are developed to such an extent that no 
direct communication is preserved between the lower and the upper parts of the visceral 
chamber, so that the calice, instead of resembling a deep tubular cup, is reduced to the 
form of a shallow basin. In general, these transversal lamine, to which the name of 
dissepiments has been given, grow from the sides of the septa in an irregular manner, and 
do not unite so as to constitute complete horizontal tabulee, extending from wall to wall ; 
but in some Corals, where the septal apparatus is even rudimentary, the bottom of the 
visceral chamber is incessantly raised by the formation of new floors or ¢abule, which 
extend horizontally through the centre of the Polypidom, and constitute, under the calices, 
a vertical series of secondary chambers. 
Intercostal dissepiments are frequently met with on the outside of the walls of the 
corallum and in compound Polypidoms, where’ the costa are highly developed, a thick 
cellular mass is thus formed, and often assumes the appearance of a cenenchyma, or common 
tissue. In other instances, the calcified derm continues to extend exteriorly without con- 
stituting distinct costae, and forms a dense or a reticulate tissue, which, in certain aggregate 
Corals, is nowhere referable to any individual Polyp, and produces a sort of intermediate 
mass or true coenenchyma. 
It is also to be remarked, that the exterior surface of most Corals is covered by a layer 
of epithelic sclerenchyma, which is sometimes thick and spongy, but in general thin and 
dense, and then constitutes a species of coating, which may be called the epitheca. 
g 
These different constitutive parts of the Polypidom furnish the principal characters 
employed in the classification of Corals ; but the mode of multiplication of the Polypi must 
also be attended to in the methodical arrangement of these Zoophytes. In some species, 
the young are only produced by the ova, and each corallum is formed by the skeleton of a 
single individual ; but in most, reproduction also takes place by fissiparity or by gemmation, 
and in those cases the young usually remain adherent to the body of their parent, and 
thus produce compound Polypidoms. The manner in which the different individual 
Polypidoms, or corallites thus united, are grouped together, varies very much, and furnishes 
also useful zoological characters. It is equally necessary not to neglect studying the 
changes which take place in the structure of Polypidoms by the progress of age. Corals, 
when young, are in general much less complicated than in the adult state, and the manner 
