210 DR. G. J. HIXDE ON SEPTASTE^A, d'oKBIGXY (1849), 



dissepiments. These are extremely thin plates, stretching, in this 

 genus, nearly horizontally across the interlocular spaces. The dis- 

 sepiments are sometimes developed at the same height in the dif- 

 ferent septal chambers of a corallite ; often, however, they are not 

 produced contemporaneously. 



For the greater portion of their length the corallite-walls and the 

 septa retain their thin, delicate characters ; but in the upper portion 

 of the coraUites, for a short distance below the surface of the corallum, 

 when the corallites have attained their limit of growth, there is a 

 deposition of solid calcareous tissue or stereoplasm, in successive 

 layers, on the interior surface of the wall, the septa, and the upper- 

 most dissepiment, which gradually fills up the upper portion of the 

 coraUites, until, as already mentioned, the fioor of the calicos is almost 

 level with the surface of the corallum, and, as calicos, they are nearly 

 obliterated. As a rule, the infilling of the corallite by stereoplasm 

 commences abruptly immediately above the uppermost dissepiments, 

 the wall and septa below these being extremely thin ; in some cases, 

 however, a partial deposit of stereoplasm takes place below the last 

 dissepiment. In longitudinal sections of the coraUites (PI. IX. fig. 14) 

 the layers of stereoplasm are clearly shown as wavy lines dipping 

 down from the sides to the centre of the interlocular spaces. Prof. 

 Duncan has described this stereoplasm as numerous, closely striated 

 dissepiments : but the mode of its deposition and its dense structure 

 appear, in my opinion, to show that it is distinct from the true 

 horizontaUy placed dissepiments. 



Pseudocolumella. — It has been already mentioned that the more 

 prominent septa extend to the centre of the coraUite, and then 

 either unite evenly by their free inner margins or curve round each 

 other to a sUght extent, thus forming a structure to which the name 

 of pseudocolumella has been given by Edy^ards and Haime* and by 

 Prof, Duncanf. In fractured surfaces and sections the character of 

 this axial structure is very distinctly shown, and there is no doubt 

 that it is produced entirely from the septal laminae, and that there 

 are no indications of a columella jpropria in the sense in which the 

 words are used by Edwards and Haime. In the upper portion of 

 the coraUites the central areas, in common with the interlocular 

 spaces, are infilled with the deposition of stereoplasm ; but this has 

 nothing in common with a genuine columeUa, which, if developed 

 at aU, would not have been Umited to the upper portion of the 

 corallites merely. 



I fully agree with d'Orbigny and Edwards and Haime, that in the 

 type species of Sej)tastrcea there is no columeUa in the proper accep- 

 tation of the word. Prof. Duncan, however, made this structure 

 one of the distinctive characters of his new genus, which was based 



* Histoire nat. des Coralliaires, vol. i. p. 61. 



+ An excellent and concise definition of this structure is thus given by Prof. 

 Dancan : — " False columella are formed by the soldering together of the inner 

 ends of two or more septa, bv the twisting of the inner ends of several septa, 

 and bv the presence of endorheca close to the septal ianer margin." — British 

 Fossil' Corals, SuppL, Pal. Soe. 1866, pt. i. p. 12. 



