264 TABULATE CORALS. 



and Haime, and they regarded it as being a " septal tooth," 

 similar to the unpaired septal ridge of certain species of Alveo- 

 lites. As pointed out by Mr R. Etheridge, jun., and myself, 

 however (Journ. Linn. Soc, vol. xiii. p. 366), this supposed 

 septal tooth was more correctly interpreted by Mr Lonsdale 

 (Geol. Russ. and Ural, vol. i. p. 95), who regarded it as being 

 an inflection of the wall of the corallite, due to its undergoing 

 the process of division by fission into two tubes. That this 

 view is the correct one is shown by the fact that this inwardly- 

 projecting ridge is of variable length and of equally variable 

 occurrence. It is never present in more than a quite limited 

 number of the tubes, and It varies in size from a hardly per- 

 ceptible protuberance up to a vertical lamina extending half- 

 way or more across the tube, while it is not uncommonly faced 

 by a corresponding ridge upon the opposite side of the vis- 

 ceral chamber. Lastly, the tabula; in Chcetetes are always well 

 developed, and are invariably complete and horizontal. Mr 

 Lonsdale considered that the tabulae of Chcetetes were placed 

 at corresponding and remote levels, and separated by zones 

 in which no tabula; were developed, but Milne-Edwards and 

 Haime explicitly deny this (Pol. Foss. des Terr. Pal., p. 261). 

 So far as Chcetetes radians, Fischer, is concerned, the truth 

 seems to lie between these two extremes, for the tabulae are 

 certainly developed throughout the entire course of the coral- 

 lites at comparatively remote intervals (PI. XII., fig. 4 a) ; but 

 they are, at the same time, periodically developed at corre- 

 sponding levels at certain horizons, so that the corallum as a 

 whole readily splits into a series of concentric layers. In the 

 other species here included in the genus the tabulae appear to 

 be developed more irregularly, though they are always present. 

 As regards the zoological affinities of Chcetetes, it is not pos- 

 sible at present to reach any final conclusion. In spite of the 

 resemblance of the corallum of Chcetetes to that of some forms 

 of Favosites (such as F. Boivcrbanki, E. and H., sp.), it is quite 

 clear that there is no direct relationship between these two 

 types, if we admit that the former possesses imperforate walls. 



