Vol. 69.] THE GENUS AULOPHYLLUM. 07 



individuals than in others. The pericentral vesicles may be long- 

 and closely packed, or they may be .shorter and loosely packed 

 (compare PI. VII, fig. 3b, with PI. VI, fig. 4 b). In instances 

 where they are compacted and where the septal lamellae are also 

 crowded, the peripheral portion of the column in the horizontal 

 section simulates the outer zone of dissepiments (PI. VI, fig. 2) 

 owing to the intersection of the pericentral vesicles by the 

 lamellae. In some cases the boundary of the central column formed 

 by the vesicles is compact and distinct (PI. VI, fig. 2) ; in others, 

 it is loose and indefinite (PI. VI, fig. 3). 



The central vesicles are very numerous and densely packed in 

 some specimens ; in others they are less numerous, and a looser 

 structure characterizes the column. If the septal lamellae extend 

 well into the column, the vesicles are much dissected (PI. VII, 

 fig. 3 b) • but, if the central portion is free from lamellae, they are 

 more like concave tabulae (PI. VI, fig. 4 b). 



The character and arrangement of the septal lamellae, as seen in 

 the transverse section, differ very greatly. They may be many in 

 number or comparatively few (see PI. VI, figs. 4 « & 3) ; but, in all 

 •cases observed, their number is greater than that of the major 

 ■septa. It has already been shown that the principal lamellae 

 penetrate to a greater or less degree into the inner portion of 

 the column, and many of the shorter coalesce with these. The 

 number of minor lamellae which unite with longer individuals, the 

 distance from the margin at which the coalescence takes place, 

 and the regularity with which this occurs, all produce marked 

 -differences between the transverse sections of different individuals. 



(2) The Four Types of the Transverse Plan of the 

 Central Column. 

 These types are : — 



(a) The 'radial' type. — In this the principal lamellae are 

 relatively straight, and converge at the centre of the 

 column (PI. VI, figs. 1 & 2 ; PI. VII, fig. 1). 



•(b) The 'tortuous' type. — In this the principal lamellae are 

 flexed, but nevertheless converge towards the centre as in 

 the ' radial ' type (PI. V, figs, bc&lb; PI. VII, fig. 4). 



•(c) The ' rose' type. — In this the principal lamellae are short, 

 and consequently leave the centre of the column occupied 

 almost entirely by tabular vesicles (PI. VI, figs. 3 & 4a). 



(d) The 'retiform' type. — In this the principal lamellae 

 ramify irregularly over the middle of the column. This 

 is commonest among more southern forms (PL VIII, 

 figs. 1 & 2). 



These designs are not sharply defined, and it is often impossible 

 to assign a particular section to one rather than to another. 



Moreover, of two sections cut from the same coral only a short 

 distance apart, the one may approach one design and the other 

 another. 



