124 THALLOPHYTA. [CH. 



tubular structure in calcareous nodules obtained from a rock of 

 Ordovician age in the Girvan district of Scotland. These 

 Authors considered the tubes to be those of some Rhizopod, 

 and proposed to designate the fossil Girvanella. 



Girvanella (fig. 26). 



Messrs Nicholson and Etheridge defined the genus as 

 follows : — 



" Microscopic tubuli, with arenaceous or calcareous (?) walls, flexuous 

 or contorted, circular in section, forming loosely compacted masses. The 

 tubes, apparently simple cylinders, without perforations in their sides, and 

 destitute of internal partitions or other structures of a similar kind." 



Fig. 26. Girvanella problematic a, Eth. and Nich. Tubules of Girvanella 

 lying in various positions and surrounding an inorganic 'nucleus' or 

 centre. From a section of Wenlock limestone, May Hill, x 65 



Since this diagnosis was published very many examples of 

 similar tubular fossils have been described by several writers 

 in rocks from widely separated geological horizons. The ac- 

 companying sketch (Fig. 26), drawn from a micro-photograph 

 kindly lent to me by Mr Wethered of Cheltenham, who has 

 made oolitic grains a special subject of careful investigation, 

 affords a good example of the occurrence of such tubular 

 structures in an oolitic grain of Silurian age from the Wenlock 



