MINUTE STRUCTUEE OF STROMATOPORA AND ITS ALLIES. 207 



in this manner (as in S. nodulata, Nich.), thin vertical sections 

 afford no conclusive evidence of the existence of these connecting- 

 tubes — the cut edges of the horizontallaminae appearing as granular 

 or subreticulate continuous lines, with only occasionally faint in- 

 dications of a vertical tubule marked out by clear calcite. In 

 Syrmgostroma densa, again, where the horizontal laminae are simi- 

 larly perforate, the substance of the fossil is so dense that the 

 evidence of vertical tubules is unsatisfactorily shown by vertical 

 sections, though there are undoubted indications of their exis- 

 tence. There are also indications of the presence of similar 

 vertical tubes in Clatlirodicfyon ceUulosum, Nich. & Murie, though 

 these also are but obscurely marked. Some further evidence on 

 this point (though likewise unsatisfactory) may be derived from 

 the examination of specimens which have preserved their calca- 

 reous skeleton but have had thieir sarcode-cavities infiltrated with 

 silica. If we examine a vertical polished section of such a spe- 

 cimen by means of a strong light reflected from the surface, and 

 by the help of a lens, we can sometimes satisfy ourselves that the 

 sarcode-chambers of successive interlaminar spaces are connected 

 by perpendicular tubuli which have been filled with silica, and 

 thus take on a higher polish than the calcareous and unchanged 

 skeleton itself. In decalcified examples of similar specimens it 

 is also sometimes possible to trace minute threads of silica pro- 

 ceeding from the rounded masses of silica which represent the 

 original sarcode-chambers. This we observed with especial dis- 

 tinctness in a specimen belonging to Principal Dawson, which 

 Dr. AV. B. Carpenter was so kind as to allow us to examine. 



Another question of importance is as to whether the horizontal 

 laminaa possess any system of tubuli running horizontally, and 

 therefore approximately parallel with their surfaces, in their actual 

 substance. Our investigations on this point lead us to believe 

 that no such canal-system exists in the substance of the concen- 

 tric lamiua?. In some specimens, in which the skeleton has been 

 silicified and the calcareous filling of the chambers has been dis- 

 solved out, the cut edges of the laminae as exposed in vertical 

 sections have a cellular structure, but it cannot be said whether 

 this is due to the presence of horizontal tubules or to the exis- 

 tence of the vertical tubules before mentioned, or whether, rather, 

 it may not be due simply to imperfect silicifieation. The last view 

 seems to us most probable. Many forms, again (such as the speci' 

 of Syringostroma and Caenostroma), possess a system of bran^ 



