326 TABULATE CORALS. 



rounded by a circumferential zone of peculiarly modified tabulae. 

 The central tube may be open throughout, but it is usually 

 intersected, at remote intervals, by delicate horizontal tabulae. 

 Surrounding the central tube on all sides, and forming its walls, 

 is a zone of tabulae, which spring from the wall of the corallite, 

 and are then bent downwards so as to become parallel to the 

 long axis of the corallite, finally joining the next tabulae below. 

 There is thus formed a series of large circumferential vesicles, 

 the convexities of which are directed upwards and towards the 

 centre of the corallite. When the section does not pass accu- 

 rately through the median plane of the corallites (as it very 

 commonly does not), then it comes to intersect the exterior 

 tabulate zone, and the cut edges of the vesicular tabulae appear 

 as transverse lines and simulate ordinary tabulae ; so that in 

 most sections parts of the tubes exhibit one set of appearances, 

 and parts show the other. When examined in tangential sec- 

 tions (fig. 43, c, a), the large corallites are seen to be hexa- 

 gonal, prismatic, or subpolygonal, and in the centre of each is a 

 rounded or oval opening representing the transverse section of 

 the central tube of the corallite. This opening is surrounded 

 by a variable number of curved lines, which are tangents to 

 the margin of the median aperture, or are concentric with it, or 

 intersect one another. These lines are the cut edges of the 

 vesicular tabulae which form the exterior zone of the corallite ; 

 and when thus exhibited they closely resemble the similar 

 lines produced by the transverse section of the central tube 

 and the infundibuliform tabulae in Syringopora. 



The smaller corallites of the colony are seen in tangential 

 sections (fig. 43, c) to be wedged in among the large tubes, 

 round which they are disposed in a single row. The circle 

 thus formed is, however, rarely or never quite complete, and 

 each of the large tubes usually comes into contact at different 

 points with one, two, three, or even four of its neighbours. 

 The small corallites are angular in shape, mostly oblong or 

 trapezoidal, and are very variable in size, being always very 

 much inferior in size to the larger corallites. Vertical sec- 



