144 THALLOPHYTA. [CH. 



but there is, perhaps, no source of error which has been more 

 responsible for the creation of numerous worthless species among 

 fossil plants. 



There is, however, another category of impressions and casts 

 of common occurrence in sedimentary rocks which requires a 

 brief notice. Very many of the fossil algae described in text- 

 books and palaeobotanical memoirs have been shown to be of 

 animal origin, and to be merely the casts of tracks and burrows. 

 A few examples will best serve to illustrate the identity of many 

 of the fossils referred to algae with animal trails and with 

 impressions produced by inorganic agency. 



Dr Nathorst of Stockholm has done more than any other 

 worker to demonstrate the true nature of many of the species of 

 ChondriteSy Gruziana, Spirophyton, Eophyton, and numerous 

 other genera. In 1867 there were discovered in certain 

 Cambrian beds of Yestrogothia, 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^. 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 remark- 

 able instance of the rare preservation of a soft-bodied organism ^ 

 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 Orstd.^, one of the Glyceridae, 

 are always branched and very algal-like in form (fig. 30, 3). 



^ Linnarsson (69) PI. xi. fig. 3. There are many good specimens of this fossil 

 in the Geological Survey Museum, Stockholm. , 



2 Nathorst (81 2), and (96). ^ Nathorst (81) p. 14. 



