EE) ER ne eh a ee ee 
SOME NEW SPECIES OF MINNESOTA ALGA! WHICH 
LIVE IN A CALCAREOUS OR SILICEOUS 
MATRIX 
JosEPHINE E. TILDEN. 
(WITH PLATES VII-IX) 
DvurinG the past three seasons there have been observed 
near Minneapolis several species of algz which deserve atten- 
tion from their peculiar manner of life, since they occupy not the 
surface but the interior of rock formations. They exist, there- 
fore, under conditions of low illumination. 
In the summer of 1894 a curious incrustation was noticed 
lining the sides of an old sunken tank which had formerly been 
used in connection with a rendering factory. The tank is situ- 
ated on the eastern bank of the Mississippi river, two miles 
below this city. It is nearly forty feet square and six to nine 
feet deep, having a muddy bottom. The walls are of boards 
standing upright side by side and driven in like piles. The 
incrustation extends from the surface of the water downwards 
to a distance of perhaps three feet, where, becoming thin and 
scaly, it gradually disappears. Its thickness in 1894 was in 
the neighborhood of 2™. By the following year there was an 
increase to 6™™, and in the present season it has attained an 
average thickness of 10™™. 
The crust covering the southwest side he the tank varies in 
color. Dull and bright zruginous, steel and brownish tints 
predominate, the two latter corresponding most nearly to the 
shades c@sius and isabellinus as given in Saccardo’s Chromotaxia 
A close view of the surface shows it to be indented by very 
minute pores or depressions, which may be compared roughly 
to the markings on some of the corals and other lime secreting 
sea = i, ee ae 
mm aoe 
