Prof. Garwood — Calcareous Algw. 495 



Kolderup ^ have described knolls of crystalline limestone containing 

 'abundant remains of Rhabdoporella (formerly described as Syringo- 

 phyllum') associated with a Gastropod and coral fauna. 



We have, therefore, in Upper Ordovician times, in the North of 

 Europe, one of the most remarkable developments of algal limestones 

 met with throughout the geological succession. In North America 

 Calcareous Algse are represented in Ordovician times by Solenopora 

 compacia, which occurs in the Trenton and Black River Limestone 

 groups, whence it was originally described by Billings under the 

 name Stromatopora compacia. It therefore occurs in America at 

 about the same horizon as in Saak and Britain. 



We may also note the occurrence of Girvanella in the underlying 

 Chazy Limestone. This occurrence was originally described by the 

 late Professor H. G. Seeley ^ under the name of Strephochetus ocellatus, 

 but is now generally admitted to be a species of Girvanella.^ 



Other forms referred to this genus have also been reported by 

 Schuchert from rocks of undoubted Ordovician age on the east coast 

 of the Behring S,traits.* 



SlLTJEIAN. 



In Britain the only horizon of Silurian age at which Calcareous 

 Algae play an important part is the Wenlock Limestone. Some years 

 ago Mr. Wethered described the occurrence of Girvanella tubes in the 

 beds of this age, especially at May Hill, Purley, near Malvern, and 

 near Ledbury.* 



Foreign Silurian. 



Outside of Britain, however, we find at this period a marked algal 

 development, and this again occurs in the Baltic area, where, especially 

 in the island of Gotland, algal growths contribute largely to several 

 of the limestones and marls. It is an interesting fact that very 

 shortly after the disappearance of the various members of the 

 Dasycladacese which were so riiuch in evidence in Ordovician times, 

 we should have the remarkable development of another group of the 

 Siphonese, which, quickly reaching a maximum, built up in their turn 

 abundant calcareous deposits. IS^odules from these limestones have 

 long been known from Gotland under the name of ' Girvanella Rock ', 

 and have been recorded by StoUey® in boulders scattered over the 

 North German plain. In 1908, however. Professor Rothpletz showed, 

 in his interesting work on these Gotland deposits,'' that the forms 

 hitherto alluded to under the term ' Girvanella' were in reality referable 

 to two different genera. One of these he showed to be a new species 

 of Solenopora, to which he gave the name S. gotlandica (distinguished 

 from S. eompacta by the comparatively small dimension of the tubes, 



' Et orienterende niveau Bergenskiffe, Rhabdoporellenkalk von Kuven mid 

 Valle, Bergens Museums, Aarbog, 1897. 



- Am. Journ. Sci., vol. xxx, p. 355, 1885. 



'•^ Geol. Mag., Dec. Ill, Vol. IV, p. 226, 1887. 



■* See Haug, vol. ii, pt. i, p. j643. 

 . ^ Q.J.G.S., vol. xlix, p. 236, 1893. 



^ Schr. d. naturw. ver. f. Schleswig-Holstein, Bd. xi, 1897, and references 

 there given. 



■^ Kungl. Svenska Vet. Akad. Handl., Bd. xliii, No. v, 1908. 



