MADREPORARIA RUGOSA. 283 



Omphyma, Ca?npophyllum, Szc.) in which the septa are short and do 

 not extend far from the wall inwards. 



The development of the peripheral zone of vesicular dissepimental 

 tissue is also very variable, its extent being in inverse proportion to that 

 of the central tabulate area. It is very well developed in Cyathophylliim 

 and in the Cyathophylloid corals generally, though considerably reduced 

 in such types as Omphyma and Campophyllum. In Zaphrentis and the 

 Zaphrentoid corals in general the dissepimental vesicular zone is greatly 

 reduced, and may be practically absent, the central tabulate area being 

 greatly developed. On the other hand, in Cystiphyllum and its imme- 

 diate allies the central tabulate area is invaded by the dissepiments, and 

 the entire visceral chamber becomes filled with a vesicular tissue, the 

 central cells of which are usually of comparatively large size, and repre- 

 sent tabulae. In Palceocyclus porpita, and in some very flat discoid 

 types (such as Microcystis), the depth of the visceral chamber is in- 

 sufficient to permit of the development of dissepiments or tabulae. These 

 structures are likewise obsolete in the aberrant genus Calceola; while 

 they are very imperfectly developed in the genus Lindstrcemia, owing 

 to the fact that the lower part of the visceral chamber becomes filled by 

 secondary stereoplasma. 



As regards exothecal structures, the corallum of the Rugose corals 

 generally exhibits externally more or less conspicuous vertical ridges, 

 which are spoken of as " rugae," and which are separated by inter- 

 vening depressions or grooves. In some cases, these longitudinal 

 ridges may correspond with the outer edges of the septa within, and 

 may thus resemble the structures known in the Aporose corals as 

 " costae." In most cases, however, the ridges correspond with the 

 interseptal loculi, and thus alternate with the septa. In some cases 

 (e.g., in Pholidophyllum) Lindstrom has described the vertical ridges 

 as carrying rows of minute, overlapping, calcareous plates or scales. 

 Other superficial exothecal structures are occasionally developed 

 in Rugose corals, such as the surface-tubercles of some species of 

 Zaphrentis, the root-like processes of attachment of Omphyma, &c, 

 and the connecting-processes which unite adjacent corallites in Eri- 

 dophyllum. The remarkable opercular plates of such genera as 

 Goniophyllum and Calceola must also be regarded as of an exothecal 

 nature. It must be regarded as doubtful, however, if anything 

 which can properly be described as a" ccenenchyma " is developed 

 in any composite Rugose coral. In such types, the corallites may 

 remain isolated except at their point of origin from the colony ; or, 

 if in contact, they may either be united by the fusion of their walls, 

 or the walls may be absent and the corallites are united to one 

 another by the extension and confluence of their septa, as seen, for 

 example, in the genus Phillipsastrcea (fig. 160). 



As regards their classification, the Rugosa are here considered as a 

 mere section of the Madreporaria, instead of as a distinct order of 

 Corals, and the signification of the name has been considerably 



