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 laminae 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. 1 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 lamellae, 

 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 Sclerenchymatous Zoantharia 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 in 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 mgosa, and Zoaiitharia caulicidata. 



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 me 

 serie, torn, ix, 1848. 



