340 GEOLOGY AND PETROLOGY OF THE GREAT SERPEXTIXE BELT OP X.S.W., 



wliieli in turn are joined to the wall {see Text-fig. 10e|. Indeed, in some eases, 

 the wall is replaced by a broad zone of vesicular tissue composed of long cells 

 flattened parallel to the outer surface (see Text-fig. lOd). The septa spring from 

 the inner side of this zone. In regard to the illustrations, it should be noted that 

 the originals of Text-flgures 10 a and d are cut from the same eorallum. The 

 greatest number of septa observed in a transverse section was sixty of the longer 

 cycle, and an equal number of intervening septa of the shorter cycle. No trace of 

 a fossula is present. Some variation is seen in the manner of attachment 

 of the longer septa to the columella. In some cases (Plate xviii., tig. 4) they 

 pass directly into the columella and appear to radiate from its outer margin, 

 though two or three of the septa occa-sioually coalesce before reaching the colum- 

 ella; in other cases (Plate xviii., tig. 2) they are crowded together, bent, and even 

 broken as if they had been twisted and thrust against the columella. Intermedi- 

 ate stages of partial twisting occur (e.g., Plate xviii., fig. 3), but this does nut 

 seem to be referable to the stage of growth. 



The most remarkable feature of the coral is the nature of the columella which 

 is a thick solid rod of rougldy ellijitical cross section, one end of the major axis 

 being drawn out into a sharp point, directed ])robably towards the cardinal septum 

 (Plate xviii., fig's. 2, 4). As in the case of Lithostrotion, this appears to be essen- 

 tially an enlargement of the plate joining the cardinal and counter septa. This 

 plate may be clearly seen in the centre of the columella (Plate xviii., flg-s. 4, 6) and 

 in the larger transverse sections is about three millimetres long and a fifth of 

 a millimetre in width. About it has been deposited a layer of lime carbonate 

 3 mm. thick, so that the major and minor axes of the columella are as much 

 as 9 and 5 or 6 mm. respectively. This layer is made up of fibres' 

 radiating from the original plate within, and to a less extent from 

 the extensions of this plate into the septa at either end (Plate xviii., fig. 6) . The 

 other septa also appear to be continued into the structure of the stereojilasm an<l 

 to join the primitive plate. The connection is most clear in the cases where the 

 .septa are not twisted (Plate xviii., fig. 4), in which case the darker, rather llexuous 

 fibres of the columella are continued into the centre of the septa. Between such 

 flark lines connecting with the septa are intervening dark lines, as if corre- 

 sponding to the distant septa of the secondary cycle. In other cases the 

 relation of lines to sei)ta is not so clear, and if the septa are tiuich twisted al)out 

 the columella, the connections are more or less destroyed (Plate xviii., fig. (il. 

 Sometimes the dark lines cannot be separated from the general radiating structure, 

 which closely resend)les that of oolite. At other times this resemblance is rendered 

 still more striking by the development of concentric layers in the thickening of the 

 columella (Plate xviii., fig. 4). 



The dissepiniental zone extends about tliree (|uarters of the distance, more 

 or less, from the wall to the columella. The vesicles are most closely jiacked in 

 that portion which is nearest to the columella and forms a half to a third of the 

 width of the dissepiniental zone. The outer and inner limits of this zone of 

 closely packed vesicles may have the dissepiments so thickened as to form more or 

 less continuous walls, the latter or thecal wall at, or just within, the circle toucli- 

 ing the ends of the shorter septa. Generally, however, there is little or no sign of 

 those structures. The nl)li(|ue but nearly longitudinal section (Plate xviii., fig. 5) 

 shows that the dissepiments are ranged in a steejdy descending sei'ies; tlie largest 

 vesicles in the central portion of (lie dissepimental zone are about three millimetres 

 in length, and one in width, though usually tliey are about half tliat size, and less 

 than that in the region of closest packing. 



