No. 1] REMARKS ON TWO FOSSIL LITHOTHAMNIA, 5 



parts, the transverse walls of the cells cannot be seen in the sec- 

 tion, and, therefore, are likely to be longer. In a transverse sec- 

 tion of a branch, these cells are angular, isodiametrical, about 

 11 — 18 jJ-. The perithallic cells are rectangular, the length being 

 mostly 1V2~'2, seldom 2^!^ times the breadth, sometimes sub- 

 square, or 14 (12)-29 (36) [x long by 12 (10)- 18 /x. . 



As the species wants conceptacles, it cannot be settled to 

 what genus it belongs. It appears most likely to be a Litho- 

 phyllum on account of the long and narrow hypothallic cells 

 without any defined stratification. Among fossil forms it probably 

 is most nearly related to LithotJiamnion (Lithophyllum?) ampJii- 

 roæformis RothpL, which is perhaps a form of the recent Lithoph. 

 byssoides (Lamk.) Fosl. 



It is unknown to me from what geologicai period these two 

 species date. According to a geological map the town of Namur 

 is situated in the coal formation. A little to the south there are 

 large fields of Iower levels (silure, devon), and a little to the north 

 there is a large field of older tertiary (eocene and oligocene). 

 Somewhat to N. E. occurs the chalk formation. The most an- 

 cient species of ArcJtæoUthothamnion hitherto known date from 

 the period last mentioned. 



