420 Dr. H. A. Nicholson on the Minute 



of a larger size than the average. These large tubes, how- 

 ever, possess no special peculiarities of structure to distinguish 

 them. 



In longitudinal sections of M. pulchella the corallites 

 are seen to be thin-walled, gradually thickening as they 

 approach the surface, the boundary-lines between contiguous 

 tubes being, in the outer part of their course, quite clearly 

 marked. The cavities of the tubes are crossed by remote and 

 complete tabuloe, which continue to be developed till close 

 upon the surface ; and there is no difference observable in the 

 tabulation of the clusters of large tubes as compared with that 

 of the ordinary corallites. There are no traces, either in 

 longitudinal or tangential sections, of any canals or pores 

 traversing the walls of the tubes or placing the visceral 

 chambers of contiguous corallites in direct communication. 



General Conclusions. 



Finally, in instituting a comparison between Heteropora and 

 Monticulvpora,) we may briefly review the following points : — 



(1) As to the general form of the corallum, the two genera 

 are very similar, though this point is of itself wholly without 

 significance, and the latter genus comprises types of very 

 varied shape and mode of growth. If, however, we compare 

 Heteropora with the ordinary dendroid tj pes of Monticulipora, 

 we have in both a corallum made up of slender fasciculate 

 tubes, which are nearly vertical in the centre of the branches, 

 and then curve outwards, gently or abruptly, to reach the 

 surface. In both, therefore, there are established two 

 distinct regions of the corallum, an axial and a peripheral 

 region. In both, moreover, these two regions are very 

 different in internal structure, the tubes in the axial region 

 of their course being thin-walled and polygonal, while in 

 the peripheral region their walls are thickened, and they 

 often become more or less rounded in form. In both, further, 

 it would appear that any special interstitial tubes that may be 

 present are developed in the peripheral region only, and ex- 

 tend either not at all, or to a very limited extent, into the 

 axial part of the corallum. 



(2) As regards the dimorphism of the corallum, all the 

 most characteristic and typical species of Monticulipora consist 

 of at least two, and sometimes of three, distinct sets of tubes, 

 which generally differ both in size and in internal structure, 

 and which are differently arranged in different cases. In 

 Heteropora the skeleton consists of a series of large tubes 

 surrounded by smaller interstitial tubes ; but it does not appear 

 that there is any special difference in the internal structure of 



