GENERAL AND COMPARATIVE STRUCTURE. 39 



Other, each carrying with it its own wall, a phenomenon which 

 could not be possible if the primitively duplex condition of the 

 walls of adjoining tubes had in reality been destroyed. 



In a third group of cases, embracing many of the most typi- 

 cal members of the group, such as M. ramosa, E. and H. (fig. 

 I, b), M. rugosa, E. and H. (fig. 2, c), M. G Ncalli, James, M. 

 nodtdosa, Nich., &c., there is no dark line running in the centre 

 of the partition between contiguous tubes ; and the walls thus 

 at first sight appear to be amalgamated, as they actually are 

 in ChcEtetes proper. In these cases, however (figs, i, b, and 2, 

 c), the state of matters really differs widely from that which 

 exists in Chcetetes, Fischer, since each visceral chamber is en- 

 closed by a distinct dark line or marginal ring, usually circular 

 or oval in outline, marking the original boundary of the tube, 

 and the interspaces between these dark lines are filled in by 

 sclerenchyma of a different texture and much lighter colour. 

 In these cases, therefore, it would appear that the corallites 

 are not only primitively distinct, but that in approaching the 

 surface they do not touch each other at all to begin with, or 

 only to a very limited extent, the ultimate union of the coral- 

 lites being effected by means of a secondary deposit of calcar- 

 eous matter. In such forms as these, therefore, the corallites 

 in the deeper parts of the corallum are thin-walled, closely con- 

 tiguous, and more or less polygonal ; whereas they become 

 much thickened and more conspicuously circular or oval in shape 

 as their mouths are approached. The structure of the wall is, 

 in fact, very similar in these cases to what is observable in 

 Stenopora, Lonsd., except that the thickening of the tubes is 

 uniform, and is not confined to the production of periodic rings. 



In other cases, again, the apparent amalgamation of the cor- 

 allites is carried still further, since the various visceral chambers 

 are bounded by well-marked lines, and the space between these 

 is simply filled with light-coloured sclerenchyma, which usually 

 exhibits fine concentric laminse of deposition in the immediate 

 neighbourhood of the actual tube-cavity, but seems to be abso- 

 lutely structureless just at that central point where we should 



