ALGJE AS ROCK-BUILDING ORGANISMS. 13 



" On certain anomalous organisms which are concerned in 

 the formation of some of the Palaeozoic limestones," 1 the 

 author adheres to his original opinion as to the affinities 

 of Girvanclla. Two figures accompanying his paper afford 

 a orood idea of the nature of this doubtful genus : in one a 

 section of the Craighead (Ayrshire) limestone is shown to 

 contain several small rounded or irregular nodules sug- 

 gestive of oolitic structure ; the second figure exhibits the 

 characteristic tubular structure of the Girvanella clusters. 

 Nicholson also notes the occurrence of the same fossil in 

 certain Lower Palaeozoic rocks of North America. In 1889 

 Wethered discovered the same tubular structure in Juras- 

 sic pisolites ; 2 he examined microscopic sections of Coralline 

 Oolite from the neighbourhood of Weymouth and of Pea 

 Grit from the northern Cotteswolds. In both cases the cal- 

 careous spherules were found to have a central nucleus sur- 

 rounded by rudely concentric layers of innumerable minute 

 tubuli exhibiting "vermiform twistings and turnings". 

 Such a microscopic structure in the pisolite grains was 

 naturally regarded as entirely opposed to the ordinary view 

 as to the concretionary origin of oolitic structure. Each 

 granule was shown to have resulted from the growth of 

 some organism round a central nucleus ; the organism 

 itself Wethered referred to Nicholson's genus Girvanclla. 

 By his subsequent investigations Wethered has consider- 

 ably extended the range of this fossil, and further evidence 

 has been accumulated as to its frequent connection with 

 oolitic structure. An oolitic structure is fairly common 

 in the Carboniferous limestone of Gloucestershire, and in 

 some localities Girvanella has been detected in the indi- 

 vidual spherules, the oolitic grains occasionally having a 

 nucleus in the form of a foraminiferal shell fragment, but 

 more frequently in these older rocks the central calcareous 

 fragment has been crystallised as calcite. 



The tubules of this Carboniferous species, Girvanella 

 Ducii, are wrapped round a central nucleus, and impart to 

 the spherules an appearance suggestive of concentric layers 



1 Nicholson, p. 23, fig. 5, A and B. 3 Wethered (1). 



