CCENOSIE.-E 303 



wedge-shaped cells ; in Selenastrum Reinsch the cells are crescent- 

 shaped ; in Calastrum Nag. the coenobe is spherical or cubical, com- 

 posed of a single layer of cells, and hollow in the centre. The species 

 are found occasionally in bog-pools ; although unciliated, the coenobe 

 swims freely in the water with a kind of rolling motion. Very little is 

 known with regard to the mode of reproduction ; no formation of zoo- 

 spores has been detected. Lagerheim describes the mode of formation 

 of a ccenobe of Selenastrum by cell-division within the mother-coenobe. 

 The affinities of this very beautiful family are clearly with Botryococcus 

 among the Protococcacese, and upwards with the Pandorineae. Scene- 

 desmus Mey. is very probably a primordial or a retrogressive member 

 of this family. 



Literature. 



Nageli-Gattungen einzelliger Algen, 1849, p. 97. 

 Reinsch — Algen-Flora Mittel-Franken, 1867. 

 Lagerheim — Bot. Centralbl., xii., 1882, p. 33. 

 Bennett — Journ. Jlicr. Soc, 1887, p. 13. 



FOSSIL ALG^. 



All that we know of the relationships between the animal and vege- 

 table kingdoms leads us to the conclusion that the appearance of animal 

 life, both in fresh and in salt water, must have been preceded by that 

 of aquatic vegetation ; and it is almost certain that these primeval 

 vegetable organisms must have had a structure and mode of life which 

 would classify them under the head of Alg» or of Schizophyces. 

 But, since even the largest of these organisms would probably consist 

 entirely of cellular tissue, it is not to be anticipated that their remains 

 could be handed down to us in the fossil state except in those cases where 

 the cell-walls were either silicified or impregnated or coated with lime. 



As far back as the Laurentian period, beds of graphite occur which 

 must undoubtedly have been the result of the decomposition of vegetable 

 matter, but all traces of the structure of the organisms from which it 

 has been derived are lost. The structures from the Russian coal-fields 

 described by Reinsch as the remains of algse allied to the Scytonemaceae, 

 are either the spores of Vascular Cryptogams, or, in many cases, are 

 inorganic crystallisations. Even the very earliest argillaceous deposits, 

 whether from fresh or from salt water, display long rounded trailing 

 impressions, which are believed by some writers to be the remains of 

 algae ; but it is exceedingly difficult to distinguish between these and 



