442 Prof. Garivood — Calcareous Algoe. 



we remember that it was not until the researches of Philippi were 

 published in 1837 that certain calcareous deposits were discovered to 

 be directly due to the growth of living forms of lime-secreting algae, 

 it is not surprising that only in comparatively recent years has the 

 importance of the fossil forms as rock-builders in past geological 

 formations been recognized. 



The original genera established by Philippi — namely, Lithothamnion 

 and Lithophyllum — are known now to have a wide distribution in the 

 present seas, and it is therefore natural that it was members of these 

 groups which were the first to be recognized in a fossil state in 

 Tertiary and, subsequently, in Upper Cretaceous rocks. 



Thus in 1858 Professor linger, of Vienna, showed the important 

 part played by Lithothamnion in the constitution of the Leithakalk of 

 the Vienna Basin, while seven years later Rosanoff contributed 

 further to our knowledge of Tertiary forms. In 1871 Giimbel 

 published his monograph on the " so-called Nullipores found in 

 limestone rocks", with special reference to the Z«YAo^Artwm/o« deposits 

 of the Danian or Maestriclit beds. Since then Lithothamnion has also 

 been reported from Jurassic rocks, and even from beds of Triassic age, 

 though in the latter case, at all events, the reference to this genus 

 appears to require confirmation. In this country the recognition of 

 fossil Calcareous Algae dates from a considerably later period. It will 

 be best first to review the chief genera which appear to be referable 

 to the Calcareous Algse, and afterwards to show the part they play as 

 rock-builders in the diflferent geological formations. 



Two important genera are usually recognized at the present day 

 as occurring in the British Palaeozoic and Mesozoic rocks — namely, 

 Solenopora and Girvanella — and to these I propose to add Wethered's 

 genus, Mitcheldeania, together with certain new forms from the 

 Carboniferous rocks of the North of England, which appear also to 

 be referable to this group. 



Solenopora. 



This genus was first created by Dybowski in 1877 for the reception 

 of an obscure organism, from the Ordovician rocks of Esthonia, which 

 he described under the name Solenopora spongioides and regarded as 

 referable to tlie Monticuliporoids. 



Nicholson and Etheridge in 1885 (Geol. Mag., p. 529) showed 

 that the form described by Billings in 1861 as Stromatopora compacta, 

 from the Black River limestones of North America, was in reality 

 Dj'bowski's genus Solenopora, and in all probability was specifically 

 identical with the form from Esthonia. Moreover, they considered 

 that the organism they themselves had described under the name of 

 Tetradium Peachii in 1877, from the Ordovician rocks of Girvan, was 

 also referable to Billing's species, though perhaps a varietal form. 

 Thus Solenopora compacta was shown to have a very wide distribution 

 in Ordovician times. 



Nicholson in 1888 defined the genus as including " Calcareous 

 organisms which present themselves in masses of varying form and 

 iiTegular shape, composed wholly of radiating capillary tubes arranged 

 in concentric strata. The tubes are in direct contact, and no 



