Notes . 
225 
continuity in one of Sir Joseph Hooker’s sections, or I should not 
have placed it on record. But I am not prepared to say that I saw it 
with anything like the precision which is exhibited in the section 
examined by Mr. Barber. 
I make these remarks in justice to myself. But I do not wish to 
detract in any way from the excellence of Mr. Barber s papers. He 
has placed our knowledge of Pachytheca on solid ground which it had 
never before attained, and has added to that knowledge a number of 
new facts of great importance, which he has interpreted in a very 
skilful and convincing manner. 
I may take the opportunity of saying that there is, I think, 
some little misapprehension on Mr. Barber’s part with regard to the 
curious round calcareous pebbles which occur in such profusion at 
the bottom of Lough Belvedere, near Mullingar \ I never found time 
to give these pebbles a more than superficial examination. They 
certainly deserve, however, a more minute study than I think they 
have yet received. An accumulation of them might easily form a 
rock, and the interpretation of its structure would not be easy. 
When the pebbles, which are of all sizes up to that of a filbert, are 
digested in a weak acid, the lime is removed and an algal mass 
remains of the same size and form. The bulk of this consists of a 
Rivularia : but if I remember rightly, my friend Mr. Archer, F.R.S., 
told me that he had detected other algae casually interwoven in the 
mass. But I have no recollection of observing anything confirming 
Mr. Barber’s statement that there is any symbiosis of Rivularia and 
Cladophora, or penetration of the former by the latter. 
The analogy which suggested itself to my mind between these 
curious pebbles and Pachytheca rested on a different point. The 
Mullingar Rivularia evidently has the power of incrusting itself with 
calcium carbonate, and I cannot help thinking that Pachytheca when 
a living organism must have had a similar propensity. If so, the 
incrusting material would undergo considerable change in the process 
of fossilization : but this would account for the manner in which its 
filamentous structure, which otherwise one would have expected to be 
very perishable, is so considerably preserved. 
W. T. THISELTON-DYER, Kew. 
1 Annals of Botany, III, p. 144. 
