PRESIDENTIAL ADDRESS, 463 
landica (distinguished from S. compacta by the comparatively small dimensions 
of the tubes, which are only about one quarter of the diameter of the latter 
species) ; the other he referred to his genus Spherocodium, which he had created 
in 1890 for certain forms from the Alpine Trias.** The presence here of Soleno- 
pora in beds of undoubted Silurian age is an interesting fact and would lead 
us to expect that it may also some day be met with in rocks of a corresponding 
age in Britain. 
Of the different forms of alge which occur in these Gotlandian deposits, 
perhaps the most interesting is Spha@rocodium, which occurs at several horizons 
in the succession. It first makes its appearance in the marl immediately over- 
lying the Dayia flags—approximately of Lower Ludlow age—where it occurs in 
considerable masses. Through the kindness of Dr. Munthe,®? who has made a 
special study of these beds in southern Gotland, I have been able to examine 
specimens of this interesting genus. In external appearance they resemble very 
closely nodules of Ortonella from the Lower Carboniferous rocks of the N.W. of 
England ; some of the nodules appear to have reached a diameter of 4 cms. The 
marl is overlain by sandstone and oolite, which are succeeded by an argillaceous 
limestone rich in nodules of Sphawrocodium gotlandicum and well exposed at 
Grotlingbo, where it is closely associated with oolite. Among the fossils of this 
limestone Spheerocodium itself plays the most important réle. 
In the overlying ‘Iliona Limestone,’ Spherocodium is decidedly rare and its 
place is taken by Spongiostroma. It is, however, found not infrequently forming 
_a thin crust on the surface of the nodules of Spongiostroma. In appearance, 
Spongiostroma resembles very closely the nodules of Spherocodium, showing the 
same concentric arrangement round coral fragments and a 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. In his original description of this genus from the Carboniferous 
rocks of Belgium, Giirich *°® refers it provisionally to the Protozoa, while Roth- 
pletz,* who has described two species S. balticum and S. Holmi from the Got- 
landian of Gotland, although admitting the difficulties of assigning it to any 
group of the animal kingdom, decides in favour of its hydrozoan affinities. 
As will be pointed out later, there appears to be no good reason why 
Spongiostroma may not be indirectly due to the presence of algal growths; but 
whatever may be the final position assigned to it, there can be no doubt as to its 
importance as a rock-building form in the Iliona limestone of Gotland. The wide 
extent of this algal horizon in the Upper Silurian of the Baltic area is shown 
by the abundance of boulders of these rocks scattered over Schleswig-Holstein, 
and it is probable that a careful examination will show the presence of this 
facies in the Silurian of the eastern Baltic Provinces. 
We may conclude, therefore, that the development of the Spherocodium 
beds of Gotland probably originally occupied nearly as wide an extension in the 
Baltic area as did the Rhabdoporella Limestones during the Ordovician Period. 
With regard to other occurrences of Calcareous Alge in Silurian rocks, it will 
be sufficient to note that of Girvanedla in the Silurian limestones of Queensland, 
recorded by Mr. G. W. Card in 1900,*° and more recently by Mr. Chapman 
from Victoria.** 
Quite recently Mr. R. Etheridge, jun., of Sydney,** has described ‘an 
organism allied to Mitcheldeania from the Upper Silurian rocks of New South 
Wales’; the figures given, however, and the description are not convincing 
that his identification can be accepted. The size of the tubes, which are from 
five to six times as large as those of I. gregaria, would alone appear to remove 
this organism from Mr. Wethered’s genus and also possibly from the Calcareous 
Alge. 
88 Bot. Cent., vol. LIL., p. 9, 1890. 
89 Geol. Foren. Férhandl. Stockholm, 1910, Bd. 32, H. 5, p. 1397. 
40 Mem. du Musée Roy. d Hist. Nat. de Belgique, tome III., 1906. 
4l Op. cit. (°). 
“2 Bull. Geol. Surv. Queensland, No. 12, pp. 25-32, pl. III. 1900, 
43 Rep. Austr. Assn. Adv. Sci., 1907-1908. 
44 Rec. Geol. Surv. N. 8S. Wales, vol. 8, pt. 4, 1909, p. 308, pl. 47. 
