_Solenopora compacta_ (Billings). Fig. 38. This species was originally described by Billings as _Stromatopora compacta_, and afterwards defined by Nicholson and Etheridge. The thallus forms sub-spheroidal masses, from the size of a hemp-seed to that of an orange. The external surface is lobulate; the fractured surface has a porcellaneous and sometimes a fibrous appearance, and is usually white or light brown in colour. In vertical section (fig. 38, B) the cells are elongated and arranged in a radiating and parallel fashion; they often occur in concentric layers. The cells have a diameter of about ¹⁄₁₇ mm. and possess distinctly undulating walls, as seen in a tangential section (fig. 38, A). Brown describes certain larger cells in the thallus (fig. 38, A) as sporangia[363], but it is difficult to recognise any distinct sporangial cavities in the drawing. The example figured is from the Trenton limestone of Canada; a variety of the same species has been recorded from the Ordovician rocks of Girvan in Ayrshire. There appear to be good reasons for accepting Brown’s conclusion that _Solenopora_ belongs to the Corallinaceae rather than to the Hydrozoa, among which it was originally included. After comparing _Solenopora_ with recent genera of Florideae, Brown concludes that “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[364].” [Illustration: FIG. 38. Solenopora compacta (Billings). A. Tangential section. × 100. B. Vertical section. × 50. (After Brown.)] The importance of the calcareous Rhodophyceae has been frequently emphasised by recent researches, and our knowledge of the rock-building forms is already fairly extensive. We possess evidence of the existence of species of different genera in Ordovician seas, as well as in those of the Silurian, Triassic, Jurassic, and more recent periods. It is reasonable to prophesy that further researches into the structure of ancient limestones will considerably extend our knowledge of the geological and botanical history of the Corallinaceae. Numerous fossils have been described as examples of other genera[365] of Rhodophyceae than those included in the Corallinaceae, but these possess little or no scientific value and need not be considered. E. PHAEOPHYCEAE (BROWN ALGAE). Olive-brown algae, thallus often leathery in texture, composed of cell-filaments or parenchymatous tissue, in some cases exhibiting a considerable degree of internal differentiation. The sexual reproductive organs may be either in the form of passive egg-cells and motile antherozoids or of motile cells showing no external sexual difference.