Tega NATURE 
[SEPTEMBER 25, 1913 
succeeding Lyckholm beds of the Baltic succession. 
This assemblage proves that the boulders did not 
originate on the Swedish continent, but from the 
more easterly-lying districts, probably from a part of 
the Baltic between Oeland and Estland, now covered 
by the sea. Similar boulders are also known at 
Lund in Scheenen, on Bornholm, and near Wisby in 
the north of Gotland. 
These facts appear to show that during the deposi- 
tion of the Jewe and the overlying Wesenberg and 
Lyckholm limestones an algal facies obtained which 
extended from Oeland to Estland and as far north 
as the Gulf of Bothnia. 
But even this area does not represent the full extent 
of the algal limestone facies in Upper Ordovician 
times. In Norway, Kiaer has shown by his detailed 
work in the Upper Ordovician rocks, Stage 5 of the 
Christiania district, the important part played by the 
Dasycladacea in this area. Here the Gastropod lime- 
stone in places forms a “ phytozoan limestone,"’ made 
up of Rhabdoporella, Vermiporella, and Apidium 
associated with a considerable development of oolite. 
Again at Kuven and Valle, in the Bergen district, 
Reusch and Kolderup have described knolls of crystal- 
line limestone containing abundant remains of Rhab- 
doporella (formerly described as Syringophyllum) 
associated with a gastropod and coral fauna. This 
horizon they have unhesitatingly referred to zone 5a 
of Kiaer’s sequence, and state that it may be found 
stretching from Geitero in the S.S.W. by Kuven, 
Valle, and Trengereid to Skarfen on Ostero, while 
Reusch has traced it further south to Stordo, near 
Dyviken and Vilkenes. 
We have, therefore, in Upper Ordovician times, in 
the north of Europe, one of the most remarkable 
developments of algal limestones met with through- 
out the geological succession. In North America also 
alge are represented in Ordovician times by Soleno- 
pora compacta, which occurs in the Trenton and 
Black River limestones groups, whence it was 
originally obtained by Billings. It therefore occurs 
here at about the same horizon as in Saak and 
Britain. 
We may also note the occurrence of Girvanella in 
the underlying Chazy limestone originally described 
by the late Prof. H. M. Seeley under the name 
Strephochetus ocellatus; but now generally admitted to 
be a form 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. 
The rocks of Silurian age in Britain, in which cal- 
careous alge play an important part, appear to be 
limited to the Wenlock limestone, from which Mr. 
Wethered has described the constant occurrence of 
Girvanella tubes, especially in the beds of this age at 
May Hill, at 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 lime- 
stone.” 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, in favour 
of Girvanella belonging to the calcareous algz, for he 
remarks: “IT certainly think that the forms which I 
have discovered in the Wenlock limestone seem more 
favourable to the vegetable theory of the origin of 
15 See Hang. 2, 1, 643. 
2, 1, 643 16 Q.1.G.S., xlix, p. 236, 1893. 
NO. 2291, 
VOL. 92] 
| 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, thisgis all that has been 
published up to the present time with regard to the 
occurrence of calcareous alge in British Silurian 
rocks; but I have every confidence that a more 
thorough microscopic examination of these rocks will 
reveal the presence of many other examples of this 
group. 
Foreign Silurian. 
Outside Britain at this period we find the most 
marked development of an algal facies, once more in 
the Baltic area, where, especially in the island of 
Gotland, algal growths 
several of the limestones and maris. It is an interest- 
ing fact that very shortly after the disappearance of 
the various members of the Dasycladaceze which were 
so much in evidence in Ordovician times, we have the 
marked development of another group of the 
Siphoneez, which quickly reached a maximum, 
building 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 scat- 
tered over the North German plain. In 1908, how- 
ever, Prof. 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. gothlandica (distinguished from 
S. compacta by the comparatively small dimension 
of the tubes, which are only about one quarter of the 
diameter of S. compacta, the genotype); the other he 
referred to his genus Spherocodium, which he had 
created in 1890 for certain forms from the Alpine 
Trias. The survival here of Solenopora into beds of 
undoubted Silurian age is an interesting fact and 
would lead us to expect that it may also some day be 
found in rocks of corresponding age in this country. 
Of the different forms of algze which occur in these 
Gotlandian deposits, perhaps the most interesting is 
Spherocodium. This organism occurs at several 
horizons in the succession. It first makes its appear- 
ance in the marl immediately overlying the Dayi 
flags—approximately of Lower Ludlow age—where 
Spherocodium occurs in considerable masses. Through 
the kindness of Dr. Munthe, who has made a special 
study of these beds in south Gotland, I have been able 
to examine specimens of this interesting form. In 
external appearance they resemble very closely nodules 
of Ortonella from the Lower Carboniferous of the 
north-west of England; some of the nodules appear 
to have reached a diameter of 14 in. The bed is over- 
lain by sandstone and ‘oolite, which are succeeded 
by an argillaceous limestone rich in nodules of Sphero- 
codium gotlandicum and well exposed at Grétlingbo, 
where it is closely associated with oolite. Among the 
fossils of this limestone Spherocodium itself plays a 
most important réle. : 
In the overlying Iliona limestone, Spherocodium-is 
decidedly rare, and its place is taken by nodules of 
Spongiostroma. It is, however, found not infre- 
quently forming a thin crust on some of the nodules 
of Spongiostroma, which have also been described by 
Prof. Rothpletz (op. cit.). In appearance, Spongio- 
stroma resembles very closely the nodules of Sphero- 
codium, showing the same concentric arrangement 
round coral fragments and total absence of the radial 
structure which is so characteristic of Solenopora. 
The actual systematic position of this organism, if 
organism it be, is still undecided. 
17 “ Ueber Algen und Hydrozoen in Silur von Gotland und Oesel” 
Kungl. Sven. Vet. Handl. Band 43. No. 5, 1903. 
contribute enormously to 
In his original. 
