64 me. Stanley smith on [March 1913, 



Brief Account of the Development of the Corallum 

 considered as a whole. 



(«) Stage A (nepionic stage). — This stage represents the 

 period during which the six primary septa were formed. The 

 character of the transverse section at the conclusion of this stage 

 is shown in PI. V, fig. 1. Close examination of this figure reveals 

 the presence of two very young secondary septa, growing out of 

 each of the counterlateral septa, so that it actually represents the 

 beginning of Stage B. 



(6) Stage B (Zaphrentoid stage). — The characters displayed 

 at this stage are essentially those of ZapTirentis, and doubtless 

 will prove eventually to be common to all Bugose forms. The 

 septa meet and coalesce at the centre of the corallum, and are 

 arranged in a pinnate symmetry ; the cardinal fossula is very large. 

 The vesicular tissue is of simple tabulate character. The whole 

 plan is uni-areal. The tissue is usually stout in proportion to the 

 size of the section, but even at this stage the greater development 

 of stereoplasmic thickening in the septa which originate at the 

 cardinal fossula may be observed. PI. V, fig. 2 illustrates the 

 general aspect of the transverse section at this stage, although 

 the separation of the septa at the centre denotes the beginning 

 of Stage C. 



(c) Stage C. — At this stage the separation of the conjoined 

 ends of the septa takes place, and the central column is developed. 

 The pinnate symmetry and large fossula are pronounced features. 

 The central column in the earlier phase of this stage is invaded by 

 the septa, the reduced ends of which form a fringe of lamella?. 

 The columnar vesicles are represented at first by simple arched 

 tabula?, or the earliest derivations from the same, so that the 

 middle of the column appears in the horizontal section almost void 

 of tissue. The outer zone of fine dissepiments is absent, and the 

 minor septa are absent or rudimentary. The plan is bi-areal. 

 PL Y, fig. 3 is the section typical of this stage ; fig. 4 repre- 

 sents the aspect towards the end of the stage, the septa having 

 withdrawn from the central column. 



(d) Stage D. — All the characters of Aulojohyllum are present at 

 this stage of its development, but some as yet are rudimentary. 

 The central column is free from the invasion of septa, and the 

 short septal lamella? at its periphery far exceed the septa in 

 number ; the principal lamella? are few, and the transverse section 

 presents a loose cellular appearance. 



Differentiation between the central vesicles and the pericentral 

 vesicles is complete, although the latter are short. The tabular 

 vesicles become more arched, and increase in number. The minor 

 septa and the dissepimental zone appear. The plan is, strictly speak- 

 ing, tri-areal ; but the outer zone is inconspicuous (PL V, fig. 5 a). 



