ASTERIONELLA
BLEAKLEYII.
TABELLARIA
FENESTRATA.
Under the city of Richmond, Va., there is such a deposit, varying from ten to twenty feet in depth, and extending for many miles. Some of the diatoms, especially those shaped like a boat, called Navicula, have a peculiar motion which at one time led observers to think them animals. No one knows how this motion is produced, and if you can find this out, you will make a very important discovery. The most common diatom in Cochituate water is Asterionella Bleakleyii. It resembles a star with rays, or the hub and spokes of a wheel. (Fig. 2.) This diatom is often found in abundance in the water supplies of cities. It never forms a complete circle, but grows into spirals or whorls which easily break up.
SPONGILLA FLUVIATILIS.
DESMIDIUM SWARTZII. FRONT AND SIDE VIEW.
Another diatom common in Cochituate is Tabellaria Fenestrata, which grows in ribbon-like forms. (Fig. 3.) The desmids resemble the diatoms in the geometrical character of their forms, but they have no shell of silex, and are therefore easily destroyed. They are readily distinguished at sight by the beautiful green color of the contained matter. In many of them there is a curious circulation of small particles, especially in the ends of those of a crescent or new-moon shape. This circulation can only be seen with a high power. Desmids are easily found in ponds and ditches; and there are several species in Cochituate. Among them is Desmidium Swartzii (fig. 4), and Closterium moniliferum. (Fig. 5.) Their beauty depends so much on color that they do not appear to advantage in the figures. You will find in examining the filterings of Cochituate water, many objects which have not been described in these papers, and among them many fragments of green filaments of the small plants belonging to the confervæ and the oscillatoriæ; sometimes you will find small round opaque forms of brown or green color, which are probably spores of plants of a larger growth; sometimes you will see the pollen of pine-trees which has fallen into the water and looks like three small balls fastened together; sometimes, though rarely, you may find one of those curious little creatures called water bears, or tardigrada; and you may be fortunate enough to catch a water spider.