LABECHIDyE. 333 



thin vertical sections (fig. 44, c) at once show that the surface- 

 tubercles are the summits of a series of vertical columns which 

 spring from the epitheca below, and are separated by intervals, 

 which are in general equal to their own width. As viewed in 

 sections of this kind, the columns appear to be composed of 

 homogeneous granular carbonate of lime, which often shows 

 no differentiation into parts. In many cases, however, a central 

 darker or lighter portion may be distinguished from the mar- 

 gins. In tangential sections (PI. XV., fig. 4), on the other 

 hand, the transversely-divided columns appear as so many oval 

 or rounded bodies, separated by small intervals, and their real 

 structure can now be more satisfactorily studied than in long 

 sections. It was originally believed by Dr Murie and myself 

 that the columns were absolutely and throughout solid and 

 homogeneous, and in many instances they certainly appear 

 to be so, while their upper ends seem to be generally solid. 

 In the examination, however, of a series of thin tangential 

 sections, I find that in many cases the columns really show a 

 distinct differentiation of their tissues. I find, namely, that 

 the columns exhibit in cross-section a more or less distinct 

 composition out of delicate concentric lamellae (PI. XV., fig. 4), 

 the outermost of which are occasionally denser than the others, 

 and form a distinct marginal line ; while there exists centrally 

 a larger or smaller axial area, which may be either darker or 

 lighter than the rest of the column, and may be surrounded by 

 a definite dark marginal ring. This central area appears really 

 to be a central tube, which is sometimes persistent, but in other 

 cases becomes filled up, in process of growth, with dense scler- 

 enchyma. There is thus reason to believe that the columns 

 of Labechia are primitively tubular, but that the median tube is 

 finally largely or entirely obliterated ; while I have at present 

 no positive evidence which would go to show that the axial 

 canal is ever absolutely continued to the summit of the column, 

 so as to open on the surface by a distinct aperture. As a rule, 

 the surface-tubercles appear to be absolutely solid and imper- 

 forate ; but I have been occasionally able to detect a minute 



