16 Mr. H. M. Bernard on the 



the tabula? in this latter case being represented internally by 

 vesicular dissepiments *. In these cases, then, instead of 

 there being one continuously expanding prototheca, there was 

 the usual repetition of protothecae which is so patent in the 

 Palaeozoic forms and still persisting, though disguised, in all 

 corals. Even in the Perforata with tall conical septal calicles 

 it must occasionally reappear, while in forms like Porites and 

 Ooniopora it is very marked (see above, p. 9). 



These diagrams (11 & 12) are instructive because we see in 

 the Silurian Palceocyclus that the original conical shape of the 

 prototheca was not yet quite got rid of but persisted as a kind 

 of stalk, whereas in Cyclolites it was quite flattened out. The 

 process of flattening was apparently a slow one, and we 

 may assume that the earlier forms always started from a deep 

 prototheca, however rapidly (as in the case of Palceocyclus, for 

 instance) the following protothecce may have flattened out. 

 Only in time was the flattening-out process so antedated that 

 the very first larval prototheca appeared as a flattened saucer. 

 And then, again, it was necessary to wait for the development 

 of a septal theca to take the place of the flattened prototheca, 

 before the latter could be left to grow outwards continuously 

 as a mere basal support. One factor in bringing about this 

 gradual flattening of the prototheca, as seen, for instance, in 

 Cyclolites, might perhaps be seen in the delaying of the 

 secretion of the rigid walls of the cup, which was probably 

 rendered possible in the case of those forms which produced 

 well-developed radial or septal thecge, the formation of which 

 might, in the early stages, use up the available material *j\ 



There was, therefore, apparently a long period during 

 which the rim of the prototheca was undergoing modification 

 in the direction of bending outwards and, if one may so 

 describe it, a period of uncertainty and hesitation. I am 

 convinced that the gradual steps by which the various flattened 



* Tabulae are secreted when the whole basal skin becomes detached at 

 once and secretes a new continuous floor. Dissepiments are the secretions 

 of portions of the skin coming loose at different times. We may see two 

 reasons for this partial detachment, and, these if correct, would throw 

 some light on the distribution of vesicular dissepiments : — (1) the mus- 

 cular attachments of the mesenteries buried in the skeleton may hold the 

 skin down at definite spots ; (2) the original floor becomes divided up by 

 radial septa, and thus the skin could not come off in one continuous 

 sheet. 



In Cyclolites the rims of the tabulae, the internal parts of which are 

 broken up into vesicular dissepiments, can be traced round the coralhun 

 as sharp lines (see fig. 12). 



t Lacaze-Duthiers, /. c, found that the septa could be the first skeletal 

 elements produced in developing Perforates, whereas phylogeneticall v the 

 prototheca came first. 



