The present contribution to the local flora is intended as an introduction to more extended research. The study is of advantage in relation to the life history of aquatic animals, the determination of ocean currents, as proved by polar discoveries, the investigation of geological strata where other fossil forms are absent, and the analysis of water supply; and, when we consider the universal distribution of diatomaceæ in the earth, the water and even in the air and the enormous deposits formed i

The few species of diatoms first discovered were included by Lyngbye, Dillwyn, and others in the genus Conferva . In 1824, the species, increased to forty-eight, were separated by Agardh into eight genera distinguished partly by their mode of growth. But little change was made until Heiberg, in 1863, advocated the division into symmetrical and asymmetrical forms. Without entering upon a general review of the later classifications, including Pfitzer's and Petit's divisions according to the number

The cell membrane is composed of two usually equal parts, each of which consists of a valve and a girdle or zone formed of cellulose modified by silica deposited in an insoluble state from a very dilute aqueous solution. The valves are more siliceous and robust than the girdle. Both are in most species easily separable, or at least the bands of the girdle which may be more or less closely fastened to the valves have a motion over each other permitting the cell to enlarge at pleasure. The longitu

The growth of diatoms follows the usual method of cell division as described by Sachs (Text Book of Botany, 2nd ed., p. 16): "The nucleus of a cell which is about to divide becomes broader, assuming the form of a biconcave lens, and its nucleolus breaks up into irregular granules which together with its other granular contents begin to form a nuclear disc in the equatorial plane. A delicate striation is now apparent in what is becoming the long axis of the nucleus, at right angles to the nuclear

The process of reproduction has been observed in many cases, but the conclusions reached are somewhat at variance with each other. The auxospore formation is simply a method of rejuvenescence. When, however, the auxospores are thrown off from filamentous diatoms, it is probable that two may conjugate, their contents dividing each into two daughter cells which unite into two zygospores. The usual method is the union of two frustules, which, throwing off the old valves, coalesce into a single mass

It may be assumed that diatoms originated in the sea; to deny this requires evidence of the existence of fresh-water species previous to the Miocene period which is entirely marine. In those subject to fluctuations of the waves, as pelagic diatoms, their existence appears to be contingent upon the methods by which the separate frustules can cohere. Various devices, including hooks, spiral bundles, horns and processes exuding threads of plasma, exist for holding together the frustules. When marin

The erratic backward and forward movement of certain diatoms, especially those of the Naviculoid group, or the slow, rolling motion of Surirella, has been discussed in so many ways without definite conclusions that a brief statement will be sufficient. Osmosis, the amœboid movement of the coleoderm, the protrusion of protoplasm or protoplasmic threads through the raphe, the existence of actual organs of locomotion or cilia, and the lack of synchronism in the chemical action occurring at the ends

Of all forms of vegetation, the Diatomaceæ are, perhaps, the most ubiquitous. Where-ever a sufficient amount of moisture, heat and light are found, they grow. It was during the Miocene period that they first appeared, and, as marine forms, reached their greatest development, both as to size and beauty of marking, while their prevalence throughout the world in enormous quantities has been often mentioned. The Miocene beds of Richmond and Maryland continued over the Cretaceous formations of New Je

Unicellular or filamentous. Cells either free, sessile, united in filaments, immersed in a gelatinous envelope or in fronds composed of branching tubes; microscopic, enclosed in a more or less siliceous envelope (frustule), composed of two parts (valves), usually connected by an intervening band (zone or girdle). Cell contents include yellowish or brownish chlorophyll-like bodies which occur in one or several bands (placcochromatic), or as variously distributed granular masses (coccochromatic) l

1. Coscinodisceæ. —Valve not divided by rays or costæ into sectors; puncta sometimes radiate; ocelli or processes absent. 2. Actinodisceæ. —Valve with radial striæ divided into sectors: ocelli and processes absent. 3. Eupodisceæ. —Valve disc-shaped with mammiform processes or one or more ocelli. ( a ) Meloseirinæ. —Frustules short, in chains. ( b ) Coscinodiscinæ. —Frustules disc form, usually single, rarely in short chains. ( a ) MELOSEIRINÆ 1. Meloseira. —Valve punctate, with a constriction or

( a ) Triceratiinæ. —Frustule cylindrical or prismatic, with three or more sides. ( b ) Biddulphiinæ. —Frustule cylindroid; valve with ends elevated into round processes or long horns. ( c ) Anauleæ. —Valve elliptical, lunate or triangular, with internal septa. ( d ) Euodieæ. —Frustule cuneate in zone view; valve lunate. ( a ) TRICERATIINÆ (1) Ditylum. —Frustule imperfectly siliceous. Zone with numerous divisions. Valve with central spine. (2) Trinacria. —Processes with sharp spines. (dis, two,

( a ) Tabellarieæ. —Valve symmetrical with respect to both the longitudinal and transverse axes; septate, not cuneate. ( b ) Meridioneæ. —Valve symmetrical with respect to the longitudinal axis, asymmetrical to the transverse axis, cuneate, finely striated. ( c ) Fragilarieæ. —Valve of varied shape, not cuneate; costate or with transverse rows of puncta. Frustule in zone view rectangular, in valve view linear or linear-elliptical, sometimes constricted in the middle, symmetrical to both axes, no

In discussing the Naviculoid group, the general divisions of Cleve are here followed, and all diatoms having a true raphe are included. I have added the genus Epithemia and also Rhopalodia, partly because they contain a raphe of a certain kind and partly because they resemble the markings of certain of the genus Hantzschia in the following group, although in other respects there is probably no similarity. The difficulty of combining the numerous genera into groups which are naturally affiliated

The Surirelloideæ are usually understood to include the genera Surirella, Podocystis, Cymatopleura and Campylodiscus, all of which resemble each other more or less, either in having a keel or markings like the divisions of the keel in Surirella and a median line, or pseudoraphe. The genus Nitzschia also has a keel, but it does not border each side of the valve as in Surirella, being found either near one margin or between it and the centre. Certain of the Surirellæ are allied to the group Trybli

Collection and Preparation of Diatoms It is assumed that every student of the Diatomaceæ has a general knowledge of the collection, preparation, mounting and examination of material. For the novice, however, the following methods, used by the author for many years, may be of service. Collection of Fresh-water Material. —The yellow film on the inside of aquaria always contains small species. Stems of water-plants near the shores of ponds and the submerged roots, the brownish coating of rocks in s