VIl] RECOGNITION OF FOSSIL ALGAE. 147 



impressk)ns\ Dawson^ has figured two good examples of 

 Carboniferous rill-marks from Nova Scotia in his paper on 

 Palaeozoic burrows and tracks of invertebrate animals. 



The specimen represented in fig. 31 affords an example of 

 a fairly well-known fossil from the Wenlock limestone, originally 

 described by Salter as Chondrites verisimilis Salt, from Dudley^. 

 He regarded it as an alga, and the graphitic impression agrees 

 closely in form with the thallus of some small seaweeds. A 

 closer examination of the fossil reveals a curious and character- 

 istic irregular wrinkling on the graphite surface, which suggests 

 an organism of more chitinous and firmer material than that of 

 an alga. 



A similar and probably an identical fossil is described and 

 figured by Lap worth ^ in an appendix to a paper by Walter 

 Keeping on the geology of Central Wales, under the name 

 of Odontocaulis Keepingi Lap. and regarded as a dendroid 

 graptolite. In any case we have no satisfactory grounds for 

 including these fossils in the plant-kingdom. 



How then are we to recognise the traces of ancient 

 algae ? There is no golden rule, and we must admit the 

 difficulty of separating real fossil algae from markings made by 

 animal or mechanical agency. The presence of a carbonaceous 

 film is occasionally a help, but its occurrence is no sure test of 

 plant origin, nor is its absence a fatal objection to an organic 

 origin. While being fully alive to the small value of external 

 resemblance, and to the numerous agents which have been 

 shown to be capable of producing appearances indistinguishable 

 from plant impressions, we must not go too far in a purely 

 negative direction. 



An important contribution to the subject of fossil algae has 

 lately appeared by Prof, ilothpletz^ He deals more particularly 

 with the much discussed Flysch^ Fucoids of Tertiary age, and 

 while refusing to accept certain examples as fossil algae, he 

 brings forward weighty arguments in favour of including several 

 other forms among the algae. He is of opinion that most of the 



1 Vide Williamson (85). ^ Dawson (90) p. 616. » Salter (73) p. 99. 



4 Lapworth (81) p. 17G, PI. vii. fig. 7. " Kotbpletz (90). 



« A term applied to a certain facies of Eocene and Oligocene rocks in 

 Central Europe. 



10—2 



