139 
Hooker . — On P achy thee a. 
the cavity and walls is, if confirmed, a fact of great importance. 
It has escaped my own notice and that of several excellent 
observers, who had devoted much time to the study of my 
specimens. 
A very able algologist, who has most carefully examined 
the specimens, Mr. Phillips, of Shrewsbury, regards Pachytheca 
as a Rivularia with the cavity invaded by an Oedogonium. 
I had from the very first considered the possibility of such an 
explanation, but was and am deterred from accepting it from 
the difficulty of supposing that a flaccid freshwater alga of 
the nature of Rivularia could have retained its spherical form 
under fossilisation ; and by the fact of the form of the en- 
larged cells terminating the peripheral filaments towards the 
centre. 
Another genus that shows some similarity to Pachytheca 
in the disposition of its filaments is Mazaea rivularioides , 
which forms globular or sub-globular hollow bodies of 
radiately disposed branching filaments in which the cells 
enlarge towards the periphery. The cavity in this genus is, 
however, the result of the decay of the inner terminations of 
the filaments. The filaments of Mazaea further bear hetero- 
cysts on their sides, of which there are no traces in Pachytheca. 
Professor Balfour has suggested that the form of Pachy- 
theca is somewhat like a Chaetophora with an incrustation of 
lime. Finally Solms-Laubach, on page 124 of his ‘ Einleitung 
in die Palaeophytologie ’ recently published, writing of Pachy- 
theca says, ‘ I entertain some doubts as to the vegetable nature 
of these remains.’ 
Such are the principal views hitherto held as to the nature 
of Pachytheca . From this it appears that until some much 
closer counterpart of the tissues of that organism than has 
hitherto been found among existing plants have been dis- 
covered, it is in vain to speculate on its near affinities ; and 
that until the peripheral termination of the filaments compos- 
ing its walls and its organs of fructification are made known, 
no certain conclusions as to its real nature and affinities are 
possible. 
