By the benevolence of Dr. Ed. Bor net I have had the op- 

 portunity of examining a piece of limestone and a ground section 

 of a fossil calcareous alga, both labelled „Litltotliamnion mar- 

 mor eum Mun.-Ovdlm. Belgique. Calcaire carbonif." On the section 

 was besides added : ,,Recu de M. Munier-Chalmas". 



I have not succeeded in finding out where this alga has been 

 described. Nor it is knovvn to Dr. Bornet or Dr. Wright. 

 I onl}^ know the following note on it in Proceedings Dublin mier. 

 Club, Nov. 19, 1880, p. 11: „Dr. E. Perceval Wright exhi- 

 bited sections of a small moi'sel of marble from the ,, Calcaire 

 Carbonifére, terrein primaire" of Namur, in Belgium, under a Y4-inch 

 objective, which clearly showed the cell-system of an alga. This 

 most ancient seaweed has been described by M. Munier-Chalmas 

 in 1876 as Lithotlnimnimi marmoreum. The wonderful state of 

 preservation of this fossil plant enabled almost the minutest details 

 of cell-structure to be seen". 



The said section and handpiece in Dr. Bornet's herbarium 

 includes, however, two different species, of which I will here give 

 a short description. Thus the calcareous alga in the section re- 

 presents an Archæolit/tothamnion, and has to be looked upon as 

 a typical A. marmoreum. In the handpiece, on the contrary, 1 

 have not found this species, but still it may occur. In two slides 

 of the same handpiece occurs another species distinguished from 

 A. marmoreum by thin fragments of terete branches, by longer 

 cells and by the want of sporangia grown in. It, therefore, is 

 likely to belong either to the genus LitJwpliyllum or to the genus 

 LiOiothamnion, probably to the former. 



