Note on calcareous alge from Michigan.! 
D. P. PENHALLOW. 
In the latter part of the summer of 1891, Sir Wm. Daw- 
son received from Mr. B. W. Thomas, of Chicago, some very 
curious and interesting calcareous pebbles formed by alge. 
They were transferred to me with the request that I should 
report upon the nature of the organisms giving rise to them. 
These pebbles were found in considerable numbers in certain 
Michigan ponds by Dr. J. W. Velei, secretary of the Chicago 
cademy of Natural Sciences. The specimens were found 
on a smooth, sandy bottom, under about four feet of water, 
and the collector states that when fresh ‘‘they had so soft 
and slippery a feeling” that he thought they were alive. Those 
Which reached me were said to be representative of the aver- 
age size. T 
thickness of about 40™™ and a diameter of about 60x 80". 
ight ashy gray, the surface was found chiefly smooth, but 
here and there with depressions of various sizes, frequently 
Upon treating a portion of a pebble with acid to remove 
ee incrustation of lime, it became evident that we had to deal 
i with an individual organism, but with a community pos- 
te Considerable diversity of plant forms, while it was 
patident that the growth of the mass had involved the in- 
ale variety of foreign bodies, some of which at least, 
Thee Served as a base for the algal growth. 
the ag my notes taken at the time, it appears that sgt 
hie eotect Condition of some of the plants, it was nee Ree 
Were MG determine them conclusively in all cases. sige 
oni Sund, however, numerous diatoms, fragments o A 
r be Cleocys tis, Calothrix and Urococcus, and in ine : 
a. Wantity, plants of Sirosiphon informe Kg. Pine po : 
A¢ fragments of coniferous wood were also noted as com 
sl desire in the prepar- 
i to a ions by Dr. Farlow in the prep 
ation of this ee valuable suggestions by 
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