GENERA OF CH^ETETID^E AND MONTICULIPORID^E. 299 



line. Interspersed between the large corallites, generally at 

 the junction of four or six of the latter, are conical or sub- 

 quadrangular eminences or tubercles (PI. XV., fig. i a), 

 placed at intervals of from a fifth of a line to half a line. 

 These eminences are the upper ends of curved and thick- 

 walled tubes, which appear to be closed above, and which are 

 not traversed by tabulae, but which are to be regarded as 

 specially modified corallites. The normal corallites are crossed 

 by horizontal, complete, and remote tabulae. 



Obs. This species seems to be distinct from D. aspera, 

 E. and H., its size apparently being less, and its form more 

 strictly dendroid ; but the latter is insufficiently described, and 

 it is quite possible that D. attrita is merely a variety of it. 

 In internal structure, tangential sections of D. attrita (PI. 

 XV., fig. i b) show that the bulk of the corallites are thin- 

 walled, polygonal, and approximately equal in size, while in- 

 terspersed among these and completely isolated by them are 

 the thick-walled tubes, which give rise to the surface-columns. 

 The apices of these columns (PI. XV., fig. i a) certainly seem 

 to be solid and imperforate ; but when divided transversely a 

 little below the surface, they are seen to be composed of dense 

 and dark-coloured sclerenchyma, deposited in successive con- 

 centric lamellae in the interior of the originally angular tube, 

 and almost always exhibiting a small circular central canal. 

 Tangential sections also show occasional small angular coral- 

 lites intercalated among the larger ones, and we may regard 

 these as spiniform corallites in which the walls are still un- 

 thickened. Vertical sections (PI. XV., fig. i c) show that the 

 corallum is mainly made up of the normal corallites, which in 

 the axis of the branches are provided with very delicate and 

 undulated walls, and are free from tabulae. As the corallites 

 curve outwards towards the surface, their walls become slightly 

 thickened, and a few remote and complete tabulae are developed 

 in their interior. The spines only extend inwards from the 

 surface to the point where the corallites bend downwards to 

 the axis of the branch, and they are seen to consist of a central 



