V] 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 visceral chamber, remains com- 

 pletely pervious from one extremity of the corallum to the other ; and the membranous 

 appendices containing the reproductive organs, and situated in the loculi, extend to its 

 basis, without encountering any obstacle • but in other species a certain number of trans- 

 verse trabiculcB or synapticults 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 lamina? occupy the same position, and subdivide the loculi into a series of small, 

 irregular 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 laminae, 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 tabulae, 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 tabula, 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 

 rorallum and in compound Polypidoms, where the costae are highly developed, a thick 

 cellular mass is thus formed, and often assumes the appearance of a ceenenchyma, 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 ceenenchyma. 



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 epit/teca. 



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, 

 (he 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 



