66 mr. Stanley smith on [March 1913, 



VI. Structural Variations. 



Unless a qualifying statement is made, the observations recorded 

 in this section refer to the adult stages of the advanced mutations 

 of the coral. The members of the group agree very closely in plan, 

 but within restricted limits vary considerably in detail. Every 

 possible variation between certain distinct types may be found. 

 Although a certain individuality usually persists throughout any 

 one corallum, yet, apart from ontogenetic changes, considerable 

 variation may exist between the successive sections of the 

 specimen. 



(1) Variation of the Component Parts of! the Corallum. 



(a) The dissepiments. — A wide zone of dissepiments is an 

 adult character, but the width attained is variable. A large 

 corallum usually shows a proportionately wide outer area, but the 

 relationship is not by any means a constant one. The zone may 

 be narrow (PI. VI, fig. 2) or broad (PI. VII, fig. 4), of uniform 

 Avidth (PI. VIII, fig. 5) or thicker at one side than another 

 (PI. VII, fig. 4). These dissepiments are much longer in some 

 specimens than in others (see PI. VI, fig. 4 b & PI. VII, fig. 3 b), 

 and often appear especially close-set near the inner margin of the 

 zone in transverse sections (PI. VII, fig. 4). 



(b) The septa. — The septa may extend almost to the centre 

 of the column, leaving only a narrow space between it and their 

 ends (PI. V, fig. 7 b), or in other cases they may leave a wide 

 one (PI. VII, fig. 5). The variations in the minor septa are not 

 so conspicuous ; but, in some cases, they project farther into the 

 medial zone of tabular vesicles than in others. The number of 

 septa present is in close relationship to the size of the corallum ; 

 in the large specimens figured in this paper there are over seventy 

 major septa, but in the small form (PI. V, fig. 9) there are only 

 forty-eight. 



(c) The tabular vesicles. — The tabular vesicles are strongly 

 arched in some of the sections examined, in others they are more 

 nearly flat. The width of the inner zone, which they occupy, does 

 not vary so much as does that of the' outer (the dissepimental) 

 zone. 



(cZ) The central column. — This, on account of its composite 

 character, affords more scope for variation than any other part of 

 the corallum. Its diameter ranges from about a quarter to nearly 

 half the diameter of the corallum. In section it may appear 

 circular or oval, corresponding to the section of the corallum, though 

 in many cases it is more oval in form than the latter. It is 

 always cuspidate, but this character is more pronounced in some 



