Dr Nathorst of Stockholm has done more than any other worker to demonstrate the true nature of many of the species of _Chondrites_, _Cruziana_, _Spirophyton_, _Eophyton_, and numerous other genera. In 1867 there were discovered in certain Cambrian beds of Vestrogothia, long convex and furrowed structures in sandstone rocks which were described as the remains of some comparatively highly organised plant, and described under the generic name _Eophyton_[226]. By many authors these fossils have been referred to algae, but Nathorst has shown that the frond of an alga trailed along the surface of soft plaster of Paris produces a finely furrowed groove (fig. 30, 2) which would afford a cast similar to that of _Eophyton_. The same author has also adduced good reasons for believing that the Eophytons of Cambrian rocks may represent the trails made by the tentacles of a _Medusa_ having a habit similar to that of _Polydonia frondosa_ Ag. Impressions of _Medusae_ have been described by Nathorst from the beds in which _Eophyton_ occurs; and the specimens in the Stockholm Museum afford a remarkable instance of the rare preservation of a soft-bodied organism[227]. By allowing various animals to crawl over a soft-prepared surface it is possible to obtain moulds and casts which suggest in a striking manner the branched thallus of an alga. The tracks of the Polychaet, _Goniada maculata_ Örstd.[228], one of the Glyceridae, are always branched and very algal-like in form (fig. 30, 3). Many of the so-called fossil algae are undoubtedly mere tracks or trails of this type. In the fossil-plant gallery of the British Museum there are several specimens of small branched casts, clearly marked as whitish fossils on a dark grey rock of Lower Eocene age from Bognor; these were described by Mantell and Brongniart[229] as an alga, but there is little doubt of their being of the same category as the track shown in fig. 30, 3. [Sidenote: FOSSILS SIMULATING ALGAE.] The well-known half-relief casts met with in Cambrian, Silurian and Carboniferous rocks, and known as _Cruziana_ or Bilobites, are probably casts of the tracks of Crustaceans. The impression left by a King-Crab (_Limulus_) as it walks over a soft surface affords an example of this form of cast. It has been suggested that some of the Bilobites may be the casts of an organism like _Balanoglossus_[230], a worm-like animal supposed by some to have vertebrate affinities. The resemblance between some of the lower Palaeozoic Bilobites and the external features of a _Balanoglossus_ is very striking, and such a comparison is worth considering in view of the fact that soft-bodied animals have occasionally left distinct impressions on ancient sediments.