VIl] GIRVANELLA. 125 



limestone of May Hill, Gloucestershire \ In the centre is a 

 crystalline core or nucleus round which the tubules have grown, 

 and presumably they had an important share in the deposition 

 of the calcareous substance. The nature of Girvanella, and still 

 more its exact position in the organic world, is quite uncertain ; 

 it is mentioned rather as a propos of the association of 

 recent Cyanophyceae with oolitic structure, than as a well- 

 defined genus of fossil algae. 



In the desciption of the calcareous nodules from Michigan, 

 Murray speaks of the Schizothrix filaments at the surface of the 

 pebbles as fairly intact, while nearer the centre only sheaths 

 were met with. It is conceivable that in some of the tubular 

 structures referred to Girvanella we have the mineralised sheaths 

 of a fossil Cyanophyceous gen us ^ The organic nature of these 

 tubules has been a matter of dispute, but we may probably 

 assume with safety that in some at least of the fossil oolitic 

 grains there are distinct traces of some simple organism which 

 was in all likelihood a plant. Some authors have suggested that 

 Girvanella is a calcareous alga which should be included in the 

 family Siphoneael As a matter of fact we must be content for 

 the present to leave its precise nature as still sub judice, and 

 while regarding it as probably an alga, we may venture to 

 consider it more fittingly discussed under the Schizophyta than 

 elsewhere. 



Wethered"* would go so far as to refer oolitic structure 

 in general to an organic origin. While admitting that a 

 Girvanella-like structure has been very frequently met with in 

 oolitic rocks, it would be unwise to adopt so far-reaching a 

 conclusion. It is at least premature to refer the formation of 

 all oolitic structure to algal agency, and the evidence adduced 

 is by no means convincing in every case. The discovery of 

 Girvanella and allied forms in rocks from the Cambrian', 



1 Wethered (93) p. 237. 



2 For figures of the sheaths of Cyanophyceous algae, see Murray (95"), 

 PI. XIX. fig. 5. Gomont (88) and (92); etc. 



^ Brown (94) p. 203. 



* For references to the papers of Wethered and others, see Seward (94), 

 p. 24. 



» E. G. Bornemann (87), PI. ii. 



