PROCEEDINGS Ol- THE COTTESWOLD CLUB 3I 



A visit to a quarry where this rock is exposed will 

 aflbrd j)root' that mollusca, corals, polyzoa, encrinites, etc., 

 were numerous in the waters of this period. This is 

 further [)roved by thin sections of the limestone which 

 show that the rock is largely made up of the debris 

 of these and other calcareous organisms in a very 

 fragmentary condition. Another interesting feature is 

 that at times these remains became either partially or 

 entirely enclosed in a crust (Fig. ia) which up to the 

 time of my investigations had passed unnoticed. To such 

 an extent has this process gone on that the crusts form 

 a very considerable part of the limestone. It therefore 

 becomes im[)ortant that we should understand the nature 

 of this growth. Microscopically examined thin sections 

 of it show that it is made u\) of endless minute tubules, 

 (Ing. IB) varying in size between '01 and '05 of a 

 millimetre in diameter. Small as these objects are, aggre- 

 gations of them ibrm dense masses of crust around the 

 calcareous remains of other organisms, but each tubule is 

 a separate individual with a clearly defined wall of 

 carbonate of lime.* Living matter occupied the interior : 

 this of course perished after death, but not so the 

 tubules which were filled in with calcite or mud, or 

 became closely pressed together. 



When first I discovered these crusts in 1889 I 

 recognised the organism as the little under.stood genus 

 Girvanella, first noticed and briefly described by Professor 

 Alleyne Nicholson and Mr R. Etheridge, junr.f Thev 

 figured one species, namely, G. problematica, but since 

 then I have discovered a considerable variety of forms 

 varying from the Silurian to the Jurassic system of rocks. 

 Minute, and at first sight apparently insignificant, as these 



'" In the illustration a I'orni of Girv.uiella is rc[)rfscntei-l not previously tigured. 



t Mon. Sil. l''oss. Girvar, pp. 22-23, !'■'• ^^■' '""'''^ recently in Nicliolson's and 

 Sydekker's Palaeontology, Vol. i, pp. 127-128. 



