208 



RUGOSA AND ALCYONARIA. 



corallum is often composite, and may be regarded as a 

 variously formed aggregate of " corallites," each of these 

 subordinate elements of the colony being essentially similar 

 in structure to the typical simple corallum. 



Fig. 98. Morphology of the Rugosa. A, Fragment of Zaphrentis gigantea, showing the 

 septa (s), with the sparse dissepiments crossing the interseptal loculi, the epitheca (e), and 

 the thin proper wall (w) ; B, Transverse section of Zaphrentis Guerangeri,-show'mg the septa 

 and dissepiments, the central area occupied solely by the tabulae, and the "fossula" (/) ; c, 

 Longitudinal section of the last, showing the arrangement of the tabulse. (A is after Edwards 

 and Haime ; B and c are after James Thomson.) 



On the other hand, there are various points in which the 

 corallum of the Rugosa differs from that of the Zoantharia 

 sclerodermata, and some of the more important of these 

 differences may be briefly alluded to here. In the first 

 place, the "septa" appear to be primitively developed in 

 four systems, so that the corallum is fundamentally con- 

 structed upon a tetrameral, instead of an hexameral type. 

 In some cases, as in Stcmria (fig. 100, A), this quadripartite 

 disposition of the septa is very conspicuous, since there are 

 four pre-eminently large septa, which form a cross in the 

 centre of the calice. In the genus Anisophyllum (fig. 100, 

 B and c) there are three of these pre-eminently developed 

 septa. In the second place, the septa usually present them- 

 selves in the adult as of two sizes only, a larger and a 

 'smaller; and their arrangement is very generally rendered 

 irregular by the presence of a singular vacant space, which 



