PORITES. 13 



It is, of course, possible that these differences may be quite independent of one another : 

 each having some cause of its own. But it is obvious that, if we can find a cause sufficient 

 to explain all three of them at once, that will in itself be some confirmation of the correct- 

 ness of the explanation. 



The conclusion so far arrived at by the author, is that the difference lies simply in the fact 

 that the trabecular, horizontal, and synapticular elements which compose tlie skeleton are thicker 

 and coarser in the Atlantic and West Indian forms than they are in those of the Indo-Pacific. 

 There seems little doubt that this will account for all the superficial differences above 

 mentioned. A glance at the magnified photographs will show the contrast in texture between 

 the calicles of the West and of the East Indian forms. Several of the latter are shown on 

 PL VII figs. 1-8, and one on PI. I. fig. 1, The last named is given because it occurs at the Cape 

 of Good Hope almost on the boundary line between the two regions, but itself belongs in 

 its character to the Indian Ocean, see p. 25. A glance at these figures enables us to state 

 that, generally speaking, the Indo-Pacific Porites show greater delicacy of the skeletal 

 elements ; that they are, consequently, far more plastic ; and that the calicles, being fashioned 

 out of a finer filigree, can not only assume a vastly greater number of shapes, including 

 the proliferations of the walls seen in the ccenenchymatous forms, but allow also of greater 

 perfection of pattern. 



This delicacy of skeletal texture can sometimes be seen better in the sections, in the 

 trabeculse and the horizontal elements, or in the fluent networks which they form together. 

 In the West Indian forms the trabeculte and horizontal elements are mostly coarse and thick, 

 cf. PI. I. fig. 56. Even when the surface elements of the calicles appear delicate, the delicacy 

 may go but a very short way down, the surface threads will often be found tg be standing up 

 upon what look like flat flakes, but are in reality the elements themselves thickening rapidly. 

 Such delicate surface skeletons are thin, as if arranged out of tissue resembling the wire- 

 edges on blunt knives. 



The fact, then, that the skeletal elements of the West Indian Porites are as a rule thicker 

 and coarser than those of the Indo-Pacific is a matter of observation. That this difference will 

 account for the other differences above referred to — the coarseness and want of symmetry in the 

 calicle patterns, the scantiness of wall proliferations, and the stiffness in growth-form — hardly 

 needs discussing. But whether tliis is the only cause I should not like to assert. Nor, again, 

 can I account for the fact that the Porites of this region are so distinguished. If it were in 

 the composition of the sea-water, one would expect to find some equally subtle differences 

 between the members of other genera which have representatives in the two regions, perhaps also 

 between the calcareous sponges, and even between the Molluscs. But, as far as I can gather, 

 nothing of the kind has ever been noticed. We have, then, to leave the fact for the student of 

 the future. 



Coming now to the morphology of the West Indian forms themselves, we must not deceive 

 ourselves into believing that, because this one common character separates them from the great 

 mass of the Indo-Pacific forms, therefore we can treat them all as one, or perhaps two, 



