88 Dr. II. A. Nicholson on the 



Chatetes raniosits, Edwards & Ilairae. 



Chatetes Dalei, Nicb. op. cit. p. H>2, pi. ill. figs. 1,1a. 



The form v,hich I described as C. Dalei, E. & H., seems to 

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

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

 C. rtifjo.sm, E. & II. ; and it would seem to be probable that 

 these two forms are no more than strongly marked varieties 

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

 nothing more than a smooth variety of the same form. 



Chatetes petrojJoUtanus , Pander. 

 Chatetes petropoUtanm, Xich. 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 'charac- 

 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 Choitetes 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 tabular 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 tabulse. 

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

 at short intervals amongst the larger tubes, in wliich the tabulas 

 are very numerous and very closely set. In transverse sections 

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

 gonal in outline, usually hexagonal or pentagonal, with very- 

 thin walls. Xo 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. 

 Chatetes discoideus, Nich. op. cit. p. 206, pi. 21. figs. 15-15 c. 

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



