THALLOPHYTES, BRYINAE. 45 



in the fossil state belong to the family of Corallineae and to the section 

 Lithothamniae, which are almost the sole components of certain deposits of 

 former ages, as they now form extensive beds at the bottom of the sea. It 

 is extremely difficult to distinguish the species in the living representatives 

 of this group, and it may readily be conceived that the difficulty of dealing 

 with the fossil forms is still greater. We shall do well to follow Unger l in 

 this matter, and put them all together as Lithothamnium ramosissimum. 

 The only proof that in these nodular calcareous forms with shrub-like 

 branching we have really to do with Lithothamnium is, as Giimbel 2 shows, 

 the presence of the characteristic structure. If we find that a body of this 

 kind consists of layers of rectangular cells lying one on another as con- 

 centric shells, there is always a possibility of confounding them with families 

 of Bryozoa, at least when the outer surface is not in a perfect state of pre- 

 servation. But in that case we have an excellent aid to discrimination in 

 the fructifications, which in the Lithothamniae are formed in great numbers 

 by later overarching of adjacent tissue, and are seen in the substance of the 

 thallus as ovoid cavities into which the sporophore projects on the under 

 side in the form of a small cone. Such undoubted Lithothamniae are 

 abundant throughout the series of Tertiary deposits, and in some localities 

 they form almost the entire material of systems of layers of no inconsider- 

 able thickness, as for example in the Lower Eocene strata of Toin in the 

 department of Ariege, and in the Pliocene deposits of the Rupe Atenea at 

 Girgenti near Syracuse, where they are quarried in the famous Latomiae. 

 They compose also the limestone of Leitha near Vienna, and the granite- 

 marble of the Nummulitic rocks. They occur also in the Senonian of the 

 Petersberg near Maestricht, in the same formation at Les Martigues near 

 Marseilles, and in the pisolite limestones of Paris, and one species has been 

 clearly identified from the zone of Ammonites bimammatus in the Jura. 

 Small shrubs and bushes of the same kind and of thoroughly similar habit 

 are also found in older formations, but as the structure has not been pre- 

 served they cannot be certainly distinguished from concretions of inorganic 

 origin. I have found such objects repeatedly in the Muschelkalk of the 

 Hainberg near Gottingen, but have never been able to detect any remains 

 of structure in my sections. Lastly, the Siluro-devonian genus Nemato- 

 phycus may be mentioned in this connection, which from its anatomical 

 structure may perhaps belong to Fucaceae ; but as we shall have to return 

 to it in speaking of coniferous woods, we will reserve any further description 

 of it for that place. 



Besides the groups of Algae hitherto mentioned, there still remains a 

 large number of forms from all the formations, from the Quaternary back* 



1 Unger (2). 2 Giimbel (1), vol. i, Introd. 





