200 Dr. Alex. Brown — On Solenopora. 



Solenopora, it is most distinct in some specimens of S. eompada 

 (Fig. 1), S. lithothamnioides (PI. V. Fig. 2), and S. fusiformis 

 (PI. V. Fig. 4), in which species the arrangement is quite com- 

 parable to what obtains in Amphiroa (Fig. 11). To the naked eye 

 mostly all these species appear to have these concentric rings. Under 

 the microscope, however, this appearance, in many cases, is not so 

 distinct, and the dark rings do not correspond with the junction 

 between neighbouring concentric rows of cells ; but there is a 

 thickening over a wider area (see PI. V. Fig. 3). This is really the 

 condition we observe in specimens of Nullipores (c/. PI. Y. Fig. 1 

 and Fig. 8). In some cases the cause of the concentric markings 

 is partly due to the concentric areas of secondary crystallization 

 (e.g. in Solenopora Jurassica). Again, the appearance and structure 

 of the cell-walls are very similar in Solenopora Jurassica and 

 Corallina mediterranea. In both, the transverse cell-walls are 

 concave on the peripheral side (Figs. 4 and 10). In the case of 

 Solenopora lilhothamnioides and Lithothamnion, each concentric layer 

 is composed of numerous cells arranged in parallel rows (cf. PI. V. 

 Figs. 1 and 2), the main difference between the two forms consisting 

 in the fact that the cell-walls of the latter are thickened over a 

 wider area, whereas those of the former are thickened only along 

 each of the concentric lines. In Solenopora compacta we find the 

 sub-dichotomous and sub-trichotomous origin of peripheral cells, 

 referred to in the first part of my paper. This method of increase is 

 characteristic of Corallina mediterranea and Lithothamnion [cf. Figs. 

 1 and 10). It probably also occurs in Solenopora Jurassica and 

 Solenopora nigra. In all the forms mentioned, both fossil and 

 recent, increase in number of cells is more common by longitudinal 

 than by transverse fission. It is pointed out by A. Eothpletz 

 (op. cit. supra p. 6) that longitudinal fission is quite characteristic 

 of certain extinct forms of Lithotliamnion, and that in these this is 

 the ordinary method of growth. Thus we have a distinct likeness 

 in the mode of cell-division between the genus Solenopora and 

 the calcareous Algse. Lastly, account must be taken of the form 

 of cells found in S. fusiformis and in S. dendriformis. In the 

 former the cells lying in each concentric layer have a fusiform 

 appearance, and to a greater or less extent dovetail into each 

 other (PI. V. Fig. 4). This condition is of very common occur- 

 rence in the vegetable kingdom ; and, with the next point to be 

 mentioned, affords a strong proof of the cellular nature of the 

 genus. In S. dendriformis the cells in cross section show a peculiar 

 branching condition, mostly in a horizontal direction, the branches 

 filling up all the interspaces between the bodies of the cells. This 

 structure forcibly reminds one of what is characteristic of certain 

 Algae, in which certain cells of the thallus assume a branching habit, 

 e.g. in Fucus. 



Thus the forms of the cells and cell-walls, the method of increase, 

 and the arrangement of the tissue-cells in the various species of 

 Solenopora bear strong evidence of relationship between that genus 

 and the calcareous Algae. 



