VIl] FOSSILS SIMULATING ALGAE. 145 



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 Upper. 

 Greensand age from Bognor ; these were described by Mantell 

 and Brongniart^ as an alga, but there is little doubt of their 

 being of the same category as the track shown in fig. 30, 3. 



The well-known half-relief casts met with in Cambrian, Silu- 

 rian 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^ , 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 Balano- 

 glossus is very striking, and such a comparison is worth consider- 

 ing in view of the fact that soft-bodied animals have occasionally 

 left distinct impressions on ancient sediments. 



The literature on the subject of fossil algae versus inorganic 

 and animal markings is too extensive and too wearisome to 

 consider in a short summary ; the student will find a sufficient 

 amount of such controversial writing — with references to 

 more — in the works quoted below^ 



In the Stockholm Museum of Palaeobotany there is an 

 exceedingly interesting collection of plaster casts obtained by 

 Dr Nathorst in his experiments on the manufacture of fossil 

 •algae,' which afford convincing proof of the value and 

 correctness of his general conclusions. 



The pressure of the hand on a soft moist surface produces a 

 raised pattern like a branched and delicate thallus. The well- 

 known Oldhamia antiqua Forbes and Oldhamia radiata Forbes*, 

 from the Cambrian rocks of Ireland may, in part at least, owe 

 their origin to mechanical causes, and we have no sufficient 



' ManteU (33) p. 166. Vide also Morris (54) p. 6. ^ Bateson (88). 



" Nathorst (81), (86) &o. Dawson (88) p. 26 et seq. Dawson (90) Delgado (86) 

 Williamson (85) Hughes (84) Zeiller (84) Saporta (81) (82) (84) (86) Fuohs (95) 

 Eothpletz (96). * Kinahan (58). 



10 



