GENERAL AND COMPARATIVE STRUCTURE. a1 



have usually been separated from Monticulipora under the 

 generic title of Dekayia, E. and H. In these cases the sup- 

 posed spines are very much reduced in number, but they are 

 quite exceptionally developed, and they constitute the well- 

 known surface-projections which are characteristic of the genus. 

 These surface -projections certainly seem to be imperforate 

 at their apices, but thin sections demonstrate conclusively that 

 they are hollow internally, and that they only differ from the 

 ordinary corallites in the greater thickness and density of their 

 walls and the apparent general absence of tabulee. I do not 

 myself entertain any doubt as to these being a peculiar form of 

 corallites — doubtless tenanted in life by peculiar zooids — the 

 mouths of which became closed by secondary deposit as the 

 corallum assumed its final characters. Nor have I any doubt 

 that the spines of forms like M. moniliformis, Nich. (fig. 5), 

 M. Jamesi, Nich., M. tumida, Phill., M. gracilis, James, and 

 others, are similarly peculiarly modified corallites, the mouths 

 of which commonly become finally closed. A further evidence 

 of this is to be found in such species as M. frondosa^ D'Orb., 

 in which the spines do not necessarily appear as spines upon 

 the surface, though thin sections exhibit appearances precisely 

 similar to what has been indicated as occurring in the forms 

 alluded to above. On the contrary, the spines often remain 

 permanently open, and appear on the surface as minute thick- 

 ened apertures between the ordinary calices, so that they have 

 been both recognised and figured as a special group of corallites 

 (Ann. and Mag. Nat. Hist., sen 4, vol. xviii. p. 92, PI. V. fig. 

 1 1). It is not, of course, essential that we should suppose these 

 singular structures to be occupied by polypes ; but I think 

 them to be modified zooids in the same sense as is true of 

 the *' avicularia " of the Polyzoa, and I shall therefore speak of 

 them as " spiniform corallites." 



In most cases, the central cavities of the " spiniform coral- 

 lites" are easily recognisable in thin sections. In other in- 

 stances, however, especially in cases where these structures are 

 developed in great numbers {e.g., in M. tumida, Phill., PI. III. 



