[Illustration: 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. [Illustration: SPONGILLA FLUVIATILIS.] [Illustration: 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 _confervae_ and the _oscillatoriae_; 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. [Illustration: CLOSTERIUM MONILIFERUM.] But you will often see the _spiculae_ of the sponge, called _Spongilla fluviatilis_. They look like pins of glass, blunt at one end and pointed at the other, and are sometimes very abundant. You may have heard that this sponge has been considered the source of the occasionally bad taste and smell of Cochituate water. When it is alive, it is not disagreeable, but when it decays it imparts to the water a very unpleasant taste and odor. It certainly is one cause of the bad quality of the water, but whether it is entitled to the sole credit is still open to question.