FOSSIL ACTINOZOA. 



93 



edge to the columella, and by the other to the inner end or 

 margin of the septa. When there is no columella, they are 

 adherent to the septa, present a free edge 

 to the cavity in the axis of the corallum, 

 and arise with the septa" (Duncan). 

 The dissepiments are incomplete hori- 

 zontal plates which grow from the sides 

 of the septa, stretching from one septum 

 to another, and more or less interfering 

 with the continuity of the loculi, and 

 breaking them up into a series of cells. 

 The loculi may thus, when the dissepi- 

 ments are numerous, become more or 

 less completely vesicular. Lastly, the 

 tabula (fig. 46) are transverse plates or 

 floors running at right angles to the axis 

 of the corallite, and dividing the theca 

 into so many horizontal compartments or 

 stories, each of which is vertically subdivided by the septa, 

 when these exist. Very generally, however, the septa are 

 absent when the coral is " tabulate." 



Fig. ^.Montlivaltia 

 caryophyllata, showing 

 the greatly-developed epi- 

 theca covering the lowe 



Ooii 



rt of the coral. Great 



Fig. 46. Columnaria alveplata, showing the corallites partitioned off 

 into stories by tabulas. Silurian. 



GEMMATION AND FISSION AMONGST CORALS. Compound 

 Corals, as before remarked, are produced by a process of 

 budding (gemmation) or cleavage (fission) from an originally 

 simple form, or by a combination of both these processes. 

 Most commonly, the compound sclerodermic coral consists of 

 a number of " corallites," each produced by a separate polype, 

 and of a common calcareous basis, or " ccenenchyma," secreted 

 by the coenosarc. There may, however, be no coenosarc, and 



