Fig. 27 B, which is copied from one of Bornet and Flahault’s drawings, represents a piece of Solen shell riddled with small canals made by the organism which has been named by the French authors _Ostracoblabe implexa_, and regarded by them as a fungus. Fig. 27 C represents a small piece of the vegetative body of _Ostracoblabe_ obtained from a decalcified shell. In endeavouring to determine the organism which has produced borings in fossil corals or shells, it must be borne in mind that some forms of canals or passages may have been the work of perforating sponges, but these are larger in diameter than those made by algae or fungi. By some writers[202] the tubular cavities in shells have been referred to true algae, but others consider them to be of fungal origin. As an example of a fossil alga referred to the Cyanophyceae, the genus _Zonatrichites_[203] may be quoted. Bornemann, who first described the specimens, points out the close resemblance in habit to some members of the recent Rivulariaceae. _Zonatrichites._ The author of the genus defines it as follows:— “A calcareous alga, with radially arranged filaments, forming hemispherical or kidney-shaped layers, growing on or enclosing other bodies. Parallel or concentric zones are seen in cross-section, formed by the periodic growth of the alga, the older and dead layers serving as a foundation on which the young filaments grow in radially arranged groups.” The nodules which are apparently formed by species of this genus occur in various sizes and shapes; Bornemann describes one hemispherical mass 8 cm. broad and 4 cm. thick. In some cases the organism has given rise to oolitic spherules, which in radial section exhibit the branched tubular cells spreading in fan-shaped groups from the centre of the oolitic grain. The section parallel to the surface of a nodule presents the appearance of a number of circular or elliptical tubes cut across transversely or more or less obliquely. The resemblance between the fossil and a specimen of the recent species _Zonatrichia calcivora_ Braun, is certainly very close, but it is very difficult, in the absence of material exhibiting more detailed structure than is shown in the specimens described by Bornemann, to decide with any certainty the true position of the fossil. The figures do not enable us to recognise any trace of cells in the radiating tubes. It is possible that we have in _Zonatrichites_ an example of a Cyanophyceous genus in which only the sheaths of the filaments have been preserved. In any case it is probable that this Mesozoic species affords another instance of a fossil alga which has been responsible for certain oolitic or other structures in limestone rocks. The species described by Bornemann was obtained from a Breccia near Lissau in Silesia, of Keuper age.