506 MR. H. M. BEENAED OK THE AEEI^'ITIES OP 



obtained mutual support in two ways : — (a) As tlie septa grew 

 in height, the polyp continually withdrew from its original 

 epithecal deposit and secreted series of dissepiments ; these are 

 a marked feature in many Palaeozoic corals, and still, as I believe, 

 form the true thecal wall in many Astrasids, as shown in fig. 10. 

 (I) The septa thickened at one or more points, the thickened 

 parts met across the intervening spaces and fused, displacing, in 

 so doing, the soft parts at the spots affected; this process is 

 only a further complication of the original infolding of the ecto- 

 derm for the formation of septa. 



Having thus far digressed with a view to explaining my mor- 

 phological standpoint, the details of which will be dealt with more 

 fully presently, we return to the question as to the generally 

 accepted affinity between Alveopora and Ooniopora. 



The skeleton of the parent polyp of Alveopora stands, as we 



have seen, at the lowest level shown in our diagrams, while 



Porites stands at the opposite extreme. The deep conical cup of 



Alveopora, with its trabecular septa running horizontally inwards, 



is represented in Porites by a flat saucer-shaped epitheca with the 



trabecular septa pointing upwards and united both radially and 



concentrically with one another. In this way Porites may have 



been deduced from a stage like that of fig. 6. But whereas in 



Alveopora the later internal septate skeleton is purely septate, 



in Porites the subsequent growth is septate and thecate. The 



septa in Porites, though not laminate, consist of radial series of 



vertical trabecules joined together in such a way that each 



series may perhaps be regarded as a highly perforate lamellate 



septum. Further, these trabeculse are united by synapticulae, 



that is, by bars running concentrically round the calicle, 



so that the whole skeleton is a kind of lattice-work made 



up of vertical and horizontal bars. The highest part of 



the septa comes, as in most other thecate corals, at some 



distance from the outer ends of the septa : we have thus trabecular 



costae and also an " edge zone" from which the polyps bud, the 



skeleton of each bud being formed upon the costal portion of 



its parent skeleton and thus what is called extra-calicular. 



Ooniopora appears to differ from Porites, not only in the size of 



the ealicles, but in the fact that the highest points of the septa 



run up so as to form a tall thin ring of perforated tissue round 



each fossa, whereas there is no such sudden rise of one or 



more trabeculae of each septum in Porites. There are other 



