1896. ] Cakareous Algae from Michigan. 217 
to agree as to the character of the dominant plant, and the 
idea derived from the observations of Dr. Farlow and myself, 
that there is no element of constancy in the composition of 
the flora of these pebbles, beyond the fact that two or three 
species are in excess, would thus seem to be greatly strength- 
Apart from the species found in the pebbles, but which 
could hardly have played any part in their original formation, 
twill be seen that there are present two and perhaps three 
species which, by their concurrent growth, may have produced 
em. It would be of great interest to know whether these 
large concretions are due to the growth of a single species, 
of, if to the combined growth of several species, the relative 
part which each plays. It is, therefore, to be hoped 
that some observer will watch the locality and obtain quite — 
young material which alone will afford the means of settling 
this interesting question, 
cGill University, Montreal. 
1—Vol. XXI.—No. 4. 
