496 Prof. Garwood — Calcareous Algoi. 



which are only about one-quarter of the diameter of the latter 

 species) ; the other lie 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 met with in rocks of a corresponding age in Britain. 



Of the different forms of Algoe which occur in these Gotlandian 

 deposits, perhaps the most interesting is Spherocodium, which, as shown 

 by Dr. Munthe,^ occurs at several horizons in the succession. It first 

 makes its appearance in the marl immediately overlying the Dayia 

 Flags — approximately of Lower Ludlow age — where it occurs in. 

 considerable masses. In external appearance these resemble very 

 closely nodules of Ortonella from the Lower Carboniferous rocks of 

 the North -West of England; some of the nodules appear to have 

 reached a diameter of 4 cm. The marl is overlain by sandstone and 

 oolite, which are succeeded by an argillaceous limestone rich in 

 nodules of Spherocodium gotlandicum and well exposed at Grotlingbo, 

 where it is closely associated with oolite. Among the fossils of 

 this limestone Spherocodium itself plays the most important role. 



In the overlying ' Iliona Limestone ' Spherocodium is decidedly 

 rare, and its place is taken by Spongiostroyna. 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. We may conclude, therefore, that the development of the 

 Spherocodium beds of Gotland probably occupied originally nearly 

 as wide an extension in the lialtic area as did the Rhahdoporella 

 limestones during the Ordovician Period. 



With regard to other occurrences of Calcareous Algae in Silurian 

 rocks, it will be sufiicient to note that of Girvanella 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, whicli are from five to six times as large as 

 those of M. gregaria, would alone appear to remove this organism 

 from Mr. Wethered's genus and also possibly from the Calcareous Algae. 



Devonian. 

 So far as I am aware, there is only one recorded occurrence of 

 Calcareous Algae from the Devonian rocks of Britain — namely, in the 

 Hope's Nose Limestone of Devonshire, from which Mr. Wethered has 

 described aggregations of tubules resembling Girvanella, but in a very 

 poor state of preservation . 



^ Bot. Cent., vol. lii, p. 9, 1890. 



- Geol. Foren. Forh. Stock., Bd. xxxii, Hft. v, p. 1397, 1910. 



2 Bull. Geol. Surv. Queensland, 1900, No. 12, pp. 25-32, pi. iii. 



■• Kep. Austr. Assoc. Adv. Sei., 1907-8, pp. 377-86, pis. i-iii, 1908. 



^ Rec. Geol. Surv. N.S. Wales, vol. viii, pt. iv, p. 308, pi. xlvii, 1909. 



