It would be very difficult to produce satisfactory evidence for the algal nature of many of the supposed fossil algae from Cambrian rocks[247]; there has been a special tendency to recognise algal remains in the oldest fossiliferous strata, due in part no doubt to the fallacy that in that period nothing higher than Thallophytes is likely to have existed. The so-called _Phycodes_ referred to by Credner[248] as characteristic of the Cambrian rocks of the Fichtelgebirge (“Phycoden-Schiefer”) is probably of inorganic origin, and comparable to the genus _Vexillum_ of Saporta[249] and other writers, which Solms-Laubach has described as being formed every day in the soft mud of our ponds where local currents are checked by branches and other obstacles[250]. There are several good specimens of _Phycodes_ in the Bergakademie of Berlin and in the Leipzig Museum which, I believe, clearly demonstrate the absence of all satisfactory evidence of an algal origin. We may next pass to a short description of a few representative types of algae, which may reasonably be classed under definite families, and accepted as evidence possessing some botanical value. A. DIATOMACEAE (BACILLARIACEAE). This family occupies a somewhat isolated position among the algae, and is best considered as a distinct subdivision rather than as a family of the Phaeophyceae or Brown algae, with which it possesses as a common characteristic a brown-colouring matter. Single-celled plants consisting of a simple protoplasmic body containing a nucleus and brown colouring matter (diatomin) associated with the chlorophyll. The cell-wall is in the form of two halves, known as _valves_, which fit into one another like the two portions of a pill-box. The cell-wall contains a large amount of silica, and the siliceous cases of the diatoms are commonly spoken of as the valves of the individual, or the _frustules_. Diatoms exhibit a characteristic creeping movement, and are reproduced by division, also by the development of spores in various forms[251].