174 



TABULATE CORALS. 



defined dark line ; but the appearances of the area within this 

 boundary-line vary according as the section cuts the tubes at 

 the level of their thickened portions, or at that of the unthick- 



Fig. 26. — A, Two tubes of Stcnofora ovata, Lonsd., cut transversely across their thickened 

 portions, and showing the contraction of the visceral chamber by an annular deposit of 

 sclerenchyma, which is not in contact with the wall on one side ; B, Two tubes of the 

 same from the axis of a branch, cut transversely, showing their thin walls and polygonal 

 form; c, Portion of a tube of the same cut longitudinally, showing the thickening of 

 the wall, the tabulae, and one of the mural pores. Enlarged twenty-five times. Pernio- 

 Carboniferous, Queensland. 



ened intervals between the latter. In the former case, the 

 visceral chamber (fig. 26, a, and PI. IX., fig. i a) is seen to be 

 greatly contracted, and to be reduced to a comparatively small 

 rounded or subpolygonal central tube, which is in turn sur- 

 rounded by a thickened ring of sclerenchyma, which usually 

 shows distinct traces of its being composed of successively- 

 deposited concentric laminae. In the latter case there is still a 

 ring of sclerenchj^ma within the dark outer polygonal boundary, 

 but this ring is of small thickness comparatively, and the cen- 

 tral tube is wide and open — the general appearances being on 

 the whole like those presented by cross-sections of the tubes in 

 the axial portion of the corallum. In both the above cases, 

 however, whether the section cut the corallites across their 

 thickened or their unthickened portions, there are two pheno- 

 mena observable which I am at present quite unable to explain. 

 One of these consists in the fact that the ring of sclerenchyma 

 within the corallites is never in contact with the outer polyo-onal 



