No particular form of microscope is required. Any instrument having standard parts, inclination of the body to the axis, a sub-stage condenser and movable stage, will prove serviceable in nearly all investigations. For critical work, measurement of striæ and location of specimens on the slide, the large models of Bausch and Lomb leave nothing to be desired. One smaller instrument may be used for rapid examination and for selection with the mechanical finger. If the stage is supplied with a vernier, the diatoms can be located rapidly and recorded for future reference. The Zentmayer Army Hospital stand with mechanical stage is excellent. The Continental stands, convenient for laboratory work, especially in the examination of bacteria, are not so serviceable as the larger stands of American and English make. The stand especially designed by Dr. Henri Van Heurck, the celebrated Belgian naturalist, is, without doubt, admirably suited to the investigation of the Diatomaceæ. In the form of the Circuit Stage as made by Watson and Sons, of London, supplied with proper condenser and mechanical stage with vernier attachment, it has been used in the preparation of the present work with much satisfaction. The drawings have all been made with an Abbé camera lucida, a 3 mm. objective and a No. 10 eye-piece, producing a magnification of about 800 diameters. All illustrations are from actual specimens in my cabinet or, in a few instances, from slides sent me by friends. In the measurement of striæ and puncta, the number in ten microns is stated, and will be found to be approximately correct in most of the drawings, except when the number is in excess of twenty in ten microns, in which case it is impossible to represent the markings accurately on figures of the magnification adopted. All drawings are from specimens in this locality, except in a few cases mentioned in the text. {137}INDEX (SYNONYMS IN ITALICS) PAGE Achnanthes, 58 brevipes Ag., 59 coarctata (Bréb.) Grun., 59 danica (Floegel) Grun., 60 exigua Grun., 59 inflata (Kuetz.) Grun., 59 lanceolata (Bréb.) Grun., 59 linearis forma curta H.L.S., 59 longipes Ag., 58 subsessilis Kuetz., 59 Actinella, 54 punctata Lewis, 54 Actinocyclus, 26 barkleyi var. aggregata Rattr., 27 ellipticus var. delawarensis n. var., 27 moniliformis Ralfs, 27