22 Mr. H. M. Bernard on the 



and size without fear of falling over. If the rise in height is 

 slow the soft parts hanging all round down to the ground 

 may go on thickening the wall, and especially the base, 

 almost indefinitely, so as to keep the corallite nearly cylin- 

 drical. In such cases the septal ridges on the inner face of 

 the cup may be continued over the edge as ridges (casta?) or 

 as rows of (costal) spines down the outside. On the other 

 hand, as soon as the base of the prototheca is sufficiently 

 firmly fixed the corallite may grow rapidly in height as well 

 :is in width, and in so doing may drag the soft parts away 

 from contact with the ground. The latter will then persist as 

 the typical u edge-zone" or " Randplatte" round the mouth 

 of the corallite. The withdrawal of the parts that thickened 

 the base while the coral grows in size leads to the latter 

 being turbinate. 



From this point of view the typical u edge-zone" is in 

 reality a vestigial structure ; it is the remains of the perisarc * 

 which in the young stage formed the secondary fleshy foot. 

 But even as such it may continue to fulfil some useful 

 function. It will always continue to leave a layer of skeletal 

 matter on the outer face of the prototheca, thus increasing the 

 thickness and strength of the latter, and it will continue to 

 form costal ( = septal) ridges or spines. In Galaxea advan- 

 tage is taken of its gradual withdrawal from contact with the 

 ground to secrete horizontal or arched films round the base of 

 each calicle. In this way the corallites of a Galaxea colony 

 are embedded in and supported by an increasingly thick 

 layer of irregular filmy vesicular tissue f. 



We are now in a position to reconcile our statement that 

 the epitheca, as usually seen in adult corals, is the rim of the 

 protothecal cup perhaps indefinitely expanded, with the 

 appearances which have led to the text-book statement 



* I suggest this distinction between Bourne's "perisarc" and the 



" edge-zone" of Miss Ogilvie; the edge-zone is the vestigial perisarc. It 

 is important not to contuse the perisarc which hangs over the solid edge 

 of the prototheca with the sides of the polyp of a perforate coral in which 

 the prototheca has been flattened down and the septa alone form a 

 secondary internal theca, and no bagging over of soft parts ever took 

 place. 



t There is in the Natural History Museum a specimen showing a 

 group of " Caryophyllia clavus" growing on a piece of a telegraph-cable 

 fvom the Caribbean Sea (700 fath.). The individuals are near together 

 and their perisarcs have covered the intervening spaces with a chalky 

 him. Here and there in the angles made by the corallites with the sub- 

 stratum the him is raised and slopes outward and downward from the 

 sides of the coral. It is this kind of free film formation which has been 

 specialized in Galaxea. 



