88 Dr. H. A. Nicholson on the 



Chcetetes ramosus^ Edwards & Haime. 

 Cheetetes Dalei, Nich. op, cit. p. 192, pi. 21. figs. 1, 1 a. 



The form which I described as C. Dalei^ E. & H., seems to 

 be really the C. ramosus of these authors. Its internal struc- 

 ture, as shown by sections, is quite identical with that of 

 C. rugosus, E. & H. ; and it would seem to be probable that 

 these two forms are no more than strongly marked varieties 

 of a single species, whilst the true C. Dalei, E. & H., may be 

 nothing more than a smooth variety of the same form. 



Chcetetes petro^olitanus. Pander, 

 Chcetetes petropolitaniis, Nicli. op. cit. p. 204, pi. 21. figs. 14, 14 a. 



I have made sections of a considerable number of examples 

 of this variable form from the Trenton Limestone of Canada, 

 the Cincinnati Group of Ohio, and the Lower Silurian rocks 

 of Sweden ; but I have not as yet had the opportunity of 

 examining Russian specimens. So far as I have seen, the 

 internal structure of this species is very constant and icharac- 

 teristic, however widely different specimens may differ in ex- 

 ternal aspect and mode of growth. In longitudinal sections 

 (PI. V. fig. 6) the tubes are seen to differ considerably in size, 

 and they may be considered as belonging to three groups. 

 The largest tubes exhibit a peculiar phenomenon, which occurs 

 in some other species of Cluetetes as well. Each tube, namely, 

 is divided down its centre into two compartments by an irre- 

 gular, flexuous, and delicate vertical septum. On the one side 

 of this septum the tabula? are usually curved, with their con- 

 vexities directed outwards, and are tolerably numerous, and 

 often more or less oblique ; whilst on the other side the tabulse 

 are more remote, and are directed at right angles across the 

 corallites. The tubes of the second group are smaller than 

 the preceding, and are furnished with regular transverse tabulae. 

 Lastly, there is a group of small tubuli, irregularly interspersed 

 at short intervals amongst the larger tubes, in which the tabulge 

 are very numerous and very closely set. In transverse sections 

 (PI. V. tig. 6 a) the corallites are seen to be more or less poly- 

 gonal in outline, usually hexagonal or pentagonal, with very 

 thin walls. No interstitial tubuli are to be observed, though 

 these would doubtless be visible in a section cut tangential to 

 and near the external surface. 



Chcetetes discoideus, James. 

 Ch(stetes discoideiis, Nicli. op. cit. p. 206, pi. 21. figs. 15-16 e. 

 This species forms thin, flattened, concavo-convex disks, 



