190 THALLOPHYTA. [CH. 



and afterwards defined by Nicholson and Etheridge. The 

 thallus forms sub-spheroidal masses, from the size of a hemp- 

 seed to that of an orange. The external surface is lobulate ; 

 the fractured surface has a porcellanous and sometimes a 

 fibrous appearance, and is usually white or light brown in 

 colour. In vertical section (fig. 38, B) the cells are elongated 

 and arranged in a radiating and parallel fashion ; they often 

 occur in concentric layers. The cells have a diameter of about 

 Jymm. and possess distinctly undulating walls, as seen in a 

 tangential section (fig. 38, A). Brown describes certain larger 

 cells in the thallus (fig. 38, A) as sporangia ^ but it is difficult 

 to recognise any distinct sporangial cavities in the drawing. 

 The example figured is from the Trenton limestone of Canada; 

 a variety of the same species has been recorded from the Ordo- 

 vician rocks of Girvan in Ayrshire. There appear to be good 

 reasons for accepting Brown's conclusion that Solenopora belongs 

 to the Corallinaceae rather than to the Hydrozoa, among which 

 it was originally included. After comparing Solenopora with 

 recent genera of Florideae, Brown concludes that " the forms 

 of the cells and cell-walls, the method of increase, and the 

 arrangement of the tissue cells in the various species of 

 Solenopora bear strong evidence of relationship between that 

 genus and the calcareous algae^" 



The importance of the calcareous Rhodophyceae has been 

 frequently emphasised by recent researches, and our knowledge 

 of the rock-building forms is already fairly extensive. We 

 possess evidence of the existence of species of different genera 

 in Ordovician seas, as well as in those of the Silurian, Triassic, 

 Jurassic, and more recent periods. It is reasonable to prophesy 

 that further researches into the structure of ancient limestones 

 will considerably extend our knowledge of the geological and 

 botanical history of the Corallinaceae. 



Numerous fossils have been described as examples of other 

 genera^ of Rhodophyceae than those included in the Coral- 

 linaceae, but these possess little or no scientific value and need 

 not be considered. 



1 Brown A. (94) p. 147. ^ ^^i^^ p. 200, ^ ^g^ Saporta (82) p. 12. 



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