ALG.E AS ROCK-BUILDING ORGANISMS. 15 



blcmatica, associated with other species of the same genus, 

 is recorded in the Wenlock limestones of May Hill. 



Wethered considers that the most interesting result of 

 his microscopical examination of the Wenlock limestone 

 has been the discovery of " new and interesting forms of 

 Girvanella, and the fact that this organism has taken so im- 

 portant a part in building up limestones ". 1 Aggregations 

 •of G. problematica were met with in rocks of the same age 

 near West Malvern and in sections of limestone from Led- 

 bury. In summarising his results Wethered makes certain 

 suggestive remarks as to the method of formation of some 

 of the Wenlock beds in which the tubular organism is so 

 •abundant ; he writes as follows : " Fragments of organisms 

 were deposited on the sea-floor, around these the Girvanella 

 tubules collected, sometimes entirely enclosing a fragment 

 with a crust of tubules, and thus giving rise to a spherule 

 •or granule, the determination of which depended on the 

 shape of the fragment enclosed ". 2 



Without attempting any complete discussion of the re- 

 ferences by other writers to this important and almost ubi- 

 quitous fossil, we must note the fact, recently pointed out by 

 Rothpletz, 3 that Nicholson has declared himself favourable 

 to the inclusion of Girvanella among (calcareous) algae. 



It has been suggested by Clement Reid 4 that possibly 

 the calcareous tubules of Girvanella may be looked upon as 

 cylindrical sheaths in which small filamentous alga; became 

 enclosed. If such an explanation be accepted we can 

 hardly speak of the genus as a true, calcareous alga, but as 

 an alga which was able to bring about a deposition of car- 

 bonate of lime outside itself, and not actually in its own cell 

 walls. 



Another author, Dr. Brown, 5 has recently lent his sup- 

 port to the opinion, previously expressed by Rothpletz and 

 others, that Girvanella must probably be placed among cal- 

 careous algae in the family Siphonece. 



1 Wethered (5), p. 239. 2 Wethered (5), p. 246. 



3 Rothpletz (1), p. 302. ' Wethered (5), p. 248. 



5 Brown, p. 203. 



