462 TRANSACTIONS OF SECTION O. 
Rhabdoporella (formerly described as Syringophyllum) associated with a 
gastropod and coral fauna. This horizon, they have unhesitatingly referred to 
zone 5a of Kiaer’s Christiania sequence and state that it may be found stretch- 
ing from Geitero in the §8.S.W. by Kuven, Valle, and Trengereid to Skarfen on 
Osterg while Reusch has traced it further south to Stordg, near Dyviken and 
Vikenes. 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 Alge are represented in Ordovician times by 
Solenopora compacta, which occurs in the Trenton and Black River limestone 
groups, whence it was originally described by Billings under the name of 
Stromatapora compacta. 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. It was originally described by the late Professor H. M. 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 
Straits.** 
SILURIAN. 
In Britain the only horizon of Silurian age at which Calcareous Alge play an 
important part is the Wenlock Limestone. Mr. Wethered some years ago from 
the beds of this age described the occurrence of Girvanella tubes at May Hill, 
Purley, near Malvern, and Ledbury.*® Of these beds Mr. Wethered remarks : 
‘The most interesting result of the microscopic study of these rocks was the 
discovery of new and interesting forms of Girvanella and the fact that this 
organism has taken so important a part in building up the limestone.’ It may 
here be mentioned that it was whilst studying these forms in the Wenlock 
Limestone that Mr. Wethered first began to favour the suggestion of Rothpletz, 
published two years previously, that Girvanella might belong to the Calcareous 
Algee, for he remarks : ‘TI certainly think that the forms which I have discovered 
in the Wenlock Limestone seem more favourable to the vegetable theory of the 
origin of this fossil than those described in my former paper, and possibly it 
may be allied to the Calcareous Alge.’ 
So far as I can ascertain, this is all that has been published up to the 
present time with regard to the occurrence of Calcareous Alege in British 
Silurian rocks; but I have every confidence that a more thorough microscopic 
examination of these rocks will reveal the presence of other examples of this 
interesting group of organisms. 
Foreign Silurian. 
Outside of Britain we find at this period, however, 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 Dasycladacece which were so much in evidence in 
Ordovician times, we should have the remarkable development of another group 
of the Siphonee, which, quickly reaching a maximum, built up in their turn 
abundant calcareous deposits. Nodules from these limestones have long been 
known from Gotland under the name of ‘Girvanella Rock,’ and have been 
recorded by Stolley ** 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 ‘ Gir- 
vanella’ 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. got- 
8 Am. Journ. Sci., vol. XXX., 1885, p. 355. 
83 Op. cil. (19) *4 See Haug. vol. II., pt. 1, p. 643. 
8 Q.J.G.8., vol. xlix., p. 236, 1893. 36 Op. cit. (8). 87 Op, cit. (8). 
