(_a_) Asterionella formosa, vegetable; a diatom, × 312. (_b_) Pediastrum simplex, vegetable; a desmid, × 200. (_c_) Cyclotella astræa, vegetable; a diatom, × 200. (_d_) Vorticella; an animalcule, × 312. (_e_) Conferva, vegetable; “green scum,” × 40. (_f_) Epithelial cell; × 200. (_g_) Fragillaria cupucina, vegetable; a diatom, × 200. (_h_) Heteromita ovata; an animalcule, × 500. (_i_) Halteria grandinella (?); an animalcule, × 200. (_k_) Anguillula fluviatilis; a water-worm, × 312. (_l_) Amœba porrecta; an animalcule, × 200. (_n_) Dinophrys; an animalcule, × 200. (_o_) Didymoprium borreri, vegetable; a desmid, × 200. (_p_) Tabellaria fenestrata, vegetable; a diatom, × 312. (_q_) Free vorticella; an animalcule, × 200. (_r_) Coccudina costata, dividing; an animalcule, × 312. (_s_) Monas umbra; an animalcule, × 312. (_t_) Cyclidium obscissum; an animalcule, × 312. (_u_) Chilodon cucullulus; an animalcule, × 200. (_v_) Epistylis nutans; young animalcules, × 200. (_w_) Paramecium; an animalcule, × 200. (_x_) Difflugia striolata, the lorica or case; an animalcule, × 200. (_y_) Conferva; vegetable “green scum,” × 312. (_z_) Vorticella microstoma; an animalcule, × 200. (_aa_) Fragments of dyed wood, × 200. (_cc_) Gomphonema acuminatum, vegetable; a diatom, × 200. (_ee_) Arthrodesmus octocornis, vegetable; a desmid, × 312. (_ff_) Scenodesmus quadricauda, vegetable; a desmid, × 200. (_ii_) Navicula rhynchocephala (?), vegetable; a diatom, × 200. Fig. 2 represents the organisms found in the Brooklyn (Ridgwood) water supply:-- (_a_) Actinophrys sol; an animalcule, × 200. (_b_) Coccudina costata; an animalcule, × 200. (_c_) Chætonotus squamatus; hairy-backed animalcule, × 200. (_d_) Notommata; a rotiferous animalcule, × 200. (_e_) Amœba guttula; an animalcule, × 200. (_f_) Melosira orichalaea, vegetable; a diatom, × 200. (_g_) Vorticella microstoma; animalcules, × 200. (_h_) Chætonotus larus; hairy-backed animalcule, × 200. (_i_) Tabellaria flocculosa, vegetable; a diatom, × 200. The original drawings from which Plate XI. is taken were prepared by Mr. William B. Lewis, for the Metropolitan Board of Health. The presence of these organisms, however startling some of them may be in appearance, is usually not objectionable; indeed, microscopic vegetable growths are frequently of service in the purification of potable water. The more important forms of bacteria (bacilli, etc.), present minute rod-like shapes, far less impressive in appearance. Considerable difference of opinion exists in regard to the sanitary value of the results afforded by the biological examination of water. While the number of bacteria found in a given quantity of water may be of aid in the formation of an opinion as to its relative safety for domestic purposes, it should be borne in mind that these micro-organisms are almost omnipresent, being contained in the air, and in soils, and articles of food.