44G -^''c»/'- Garwood — Calcareous Algce. 



and Professor Nicholson obtained their specimens, and an examination 

 of this material has impressed me stron<;ly with the resemblance of 

 Mitcheldeania to forms such as Soleiiopora and Girvanella, now 

 usually classed among the Calcareous Algse. In the rocks in which 

 it occurs Mitcheldeania appears as rounded and lobulate nodules, 

 breaking with porcellanous fracture and showing concentric structure 

 on weathered surfaces, very similar to nodules of Solenopora ; while 

 under the microscope the branching character of the tubules and 

 their comparatively minute size appear to separate them from the 

 Monticuliporoids. Professor Nicholson appears to rely on the 

 presence of pores, which he thought he observed in the walls of 

 both the larger and finer tubes, for the inclusion of this genus with 

 the Hydrocorallines, though he appeared to be doubtful about their 

 occurrence in the interstitial tubuli. An examination of a large 

 number of slides has failed to convince me of the presence of pores, 

 even in the larger ' zooidal tubes'. The large 'oval or circular' 

 apertures noticed by Nicholson appear to be either elbows in the 

 undulating tubes cut across where these bend away from the plane 

 of the section, or places where a branch is given off from a tube at 

 an angle to the plane of the section. If this view be accepted, there 

 appears to be no sufiicient reason Avhy Mitcheldeania should not be 

 ranged with Solenopora and other similar forms, and included among 

 the Calcareous Algte — a position which its mode of occurrence and 

 general structure has led me, for some time, to assign to this organism. 



In addition to the three chief forms described above from British 

 rocks, a study of numerous thin sections from the Lower 

 Carboniferous rocks of the north-west of England has revealed the 

 presence of several distinct organisms, which will, I think, eventually 

 be found to be referable to the Calcareous Algse. 



This meagre list appears to exhaust the genera known at the 

 present time from the Lower Carbouiferous rocks of Britain, while 

 the only additional genus so far recorded from the Mesozoic and 

 Tertiary rocks of this country (if we accept Rothpletz's sub-genus 

 Solenoporella) is Chara from the Wealden beds of Sussex, the 

 uppermost Jurassic of the Isle of Wight and Swanage, and the 

 Oligocene of the Isle of Wight. 



Outside this country the literature on fossil Calcareous Algae 

 is much more extensive. The interest originally aroused on the 

 Continent by the writings of Philippi, of linger of Vienna, Cohn, 

 Rosanoff, Giimbel, Saporta, and Munier-Chalmas, has been further 

 maintained in our own time by Kornemann, Steinmann, Friih, Solms- 

 Laubach,' Rothpletz, Walther, Kiaer, and others; while the more 

 favourable conditions which obtained for the growth of these 

 organisms, especially during Silurian, Triassic, and Tertiary times, 

 has afforded a much wider field for their observation. 



Thus, in addition to the forms recorded from this country, an 

 important part has been played by members of the family of 

 the Dasycladacese, together with such genera as Spherocoditun, 

 Lithothamnion, and Lithophyllum. 



[To he contimied in our next Number.) 

 ' Fossil Botany (Oxford), 1891. 



