PORITES. 19 



the known forms and succeeded in arranging them into" groups according to these principles. 

 We have seen tliat the crude method of grouping according as the branches are what is meant 

 by clavate, furcate, and divaricate completely and confessedly breaks down. We do not pretend 

 that the methods of analysis here laid down are anything like complete. All we claim is that 

 a beginning is here made which, at least, promises to carry us some little way towards 

 a natural classification. One distinct group may, for instance, be seen in the forms above 

 mentioned (p. 17), in which the initial stock rises directly into a gradually thickening knob. 

 They all seem to show close clusters of sinuous stems forking and thickening as they rise. 

 We have apparently a progressive but irregular thickening of the successive units with 

 abortions of prongs or smooth " knee-bends." It seems safe to assert that they all differ in- 

 asmuch as their initial colonies were different. For a further analysis of the growth-forms see 

 Table III., p. 130. 



We turn lastly to the most difficult feature of the West Indian corals. Differences in 

 growth-form may for a while escape our analysis, but eventually yield to a careful comparative 

 study. The differences in the calicles are far more subtile. However, with the assistance of 

 the diagrams published in Vol. V. p. 14, and reproduced on p. 139, we can make a small 

 beginning (see further Table IV.). 



The Calicles. — We have already noticed that, as compared with the Indo-Pacific Forties, 

 the calicles of the West Indian forms are coarse and irregular, owing to the thickness 

 and consequent stiffness of the skeletal elements. It is quite possible to find Indo-Pacific 

 forms with calicles as large, coarse, and irregular as almost any West Indian form, and perhaps 

 even some of the latter whicli show something like the same delicacy and wealth of detail as 

 a few of the former ; but on the average the difference is as described. It was this precision in 

 the detail of the calicle skeleton in the Indo-Pacific forms which enabled us, as soon as the 

 principles of structure were made out, to make some approach to arranging the calicles into 

 groups according to structure. It is doubtful whether any such approximation could have 

 been made had we had the calicles of the West Indian forms alone to work with. Even the 

 fundamental formula of the septa could hardly have been made out without a previous know- 

 ledge of it obtained from our study of the Indo-Pacific forms ; while, again, the raggedness of 

 the surface skeleton makes it difficult to analyse the -wall-formation. This task was 

 comparatively easy in the case of most of the Indo-Pacific forms, and we were able to base a 

 classification of the calicles upon the number of rings of trabeculae which take part in the 

 separation of adjacent calicle-cavities. 



Applying this same system of analysis to the West Indian forms, we may note, first of all, 

 that the Diagram B, fig. 1, p. 139, which was applicable to only a small proportion of the 

 Indo-Pacific forms, viz. to those in which the walls were zigzag, represents the condition seen 

 in the vast majority of the West Indian forms. In this respect there is a uniformity in the 

 calicles which has certainly greatly added to the difficulty of classification ; nevertheless, differ- 

 ences of other kinds can generally be found : the height of the wall and the varying character 

 of its elements, the different characters of the septa, and the stiffness or fluency of the 



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