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].”
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.
With one or two exceptions all the genera are marine. They have a wide distribution at the present day, and are especially characteristic of far northern and extreme southern latitudes. The gigantic forms Lessonia, Macrocystis and others already alluded to, belong to this group; also the genus Sargassum, of which the numberless floating plants constitute the characteristic vegetation of the Sargasso Sea.
Palaeobotanical literature is full of descriptions of supposed fossil representatives of the brown algae, but only a few of the recorded species possess more than a very doubtful value; most of them are worthless as trustworthy botanical records. Many of the numerous impressions referred to as species of Fucoides and other genera present a superficial resemblance to the thallus of the common Bladder-wrack and other brown seaweeds. Such similarity of form, however, in the case of flat and branched algal-like fossils is of no scientific value. In many instances the impressions are probably those of an alga, but they are of no botanical interest. The flat and forked type of thallus of Fucus, Chondrus crispus (L.) and other members of the Phaeophyceae is met with also among the red and green algae, to say nothing of its occurrence in the group of thalloid Liverworts, or of the almost identical form of various members of the animal kingdom. The variety of form of the thallus in one species is well illustrated by the common Chondrus crispus (L.). This alga was described by Turner[366] in his classic work on the Fuci under the name of Fucus crispus as “a marine Proteus.” It affords an interesting example of the different appearance presented by the same species under different conditions, and at the same time it furnishes another proof of the futility of relying on imperfectly preserved external features as taxonomic characters of primary importance.
An example of a supposed Jurassic Fucus is shown in fig. 49, and briefly described in the Chapter dealing with fossil Bryophytes.