x BRITISH FOSSIL CORALS. 
duced by their existence must be considered as one of the most striking features of this 
zoological division. ‘The septa are developed successively, as the Polyp grows, and in general 
six of these vertical laminz constitute the primary or fundamental cyclum. Shortly after- 
wards a second circle, equally composed of six septa, appears, and the twelve loculi situated 
between these secondary septa and the primary ones are next subdivided by a third row or 
cyclum of twelve younger septa. The number of the septa often augments still more, and is 
sometimes carried very high ; but in general the primary septa continue to be more developed 
than the others, and thus divide the whole of the radiate structure into six distinct groups or 
systems. In some instances, however, the secondary, or even the tertiary, septa grow so rapidly, 
that they soon exactly resemble those of the first cyclum, and in such cases the number of the 
systems is apparently much greater.’ Sometimes the number of the primary septa is, on 
the contrary, reduced to four, or perhaps even to two, but never reaches eight, as would be 
the case if the Polypi of this order had ever eight tentacula and eight perigastric lamelle, 
a structure which is always met with in the order of Alcyonaria. It is also to be noted, 
that the septa vary considerably in their structure, and thus furnish most important 
characters, not only for the distinction of species and genera, but even for the formation of 
higher zoological divisions in this order of Polypi. 
Zoantharia may be divided into two principal groups, characterised by the structure of 
the parietes of their body. One of these sections comprises the species in which the 
dermal tissue remains soft and flexible ; the other contains those the teguments of which 
assume an osseous structure and constitute a calcareous Polypidom. 
The Scterencuymatous ZoantHarta are the only Zoophytes of this order which we 
shall have to mention in the sequel of this work ; it would, therefore, be superfluous for us 
to treat of the classification of Malacodermous Zoantharia; but it is necessary that we 
should give a detailed account of the methodical arrangement of the first of these groups. 
Little is known concerning the anatomical modifications of the soft parts m the different 
representatives of this zoological form; but the structure of the Polypidom offers great 
variety, and furnishes, to an attentive observer, data which appear sufficient for the natural 
classification of Sclerenchymatous Zoantharia. The principal characters which we have 
made use of for that purpose, are derived from the dense or porous structure of the 
sclerenchyma ; the predominance of the septal apparatus, the mural tissue or the tabular 
system in the formation of the corallum; the existence or the absence of dissepiments 
uniting the septa and subdividing the loculi, and the mode of development of the Polypi. 
Five principal divisions may be thus established in this section, and may be designated by 
the following appellations : Zoantharia aporosa, Zoantharia perforata, Zoantharia tabulata, 
Zoantharia rugosa, and Zoantharia cauliculata. 
1 The laws by which the development of the septal apparatus appears to be regulated, have been laid 
down in our memoir on the Structure of Corals, ‘published in the Annales des Sciences Naturelles, 3™° 
série, tom. ix, 1848. 
