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 Jersey have outcropped in certain localities within our district, but are not considered in this discussion.

The function of diatoms is not essentially different from that of other algÆ in providing food for aquatic animals, such as SalpÆ and oysters, but it is, however, in other respects that they are not only important but necessary factors in the preservation of life.

I am not certain if Thomson fully understood the matter, but he has remarkably described the facts. When "the vital breath" of returning spring animates the earth, the "subterranean cells" of diatoms, the "atoms organized," through the liberation of vast quantities of oxygen, immediately begin the purification of the "putrid streams." Were these streams not so purified, the accumulation of animal and vegetable dÉbris would eventually cause an enormous bacterial growth fatal to animal life.

DIATOMACEÆ

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) lining the inner walls. Growth by ordinary cell division or by auxospores; sexual multiplication by the formation of sporangia. Valves of two kinds: (a) Those in which the markings or parts are more or less concentric (CentricÆ); (b) Those (PennatÆ) in which the parts are more or less symmetrically divided by a line (pseudoraphe) or by a cleft (raphe).

CENTRICÆ

Valves without a dividing line or cleft; markings more or less radiate; transverse section of frustule circular, polygonal, or elliptical, sometimes irregular.

Divided into four groups:

1. DiscoideÆ.—Frustules (cells) discoid; valves without horns or elevations (sometimes with processes).

2. SolenoideÆ.—Frustules with numerous girdle bands.

3. BiddulphioideÆ.—Frustules box-like, i. e., with the longitudinal axis greater than in the DiscoideÆ. Valves with two or more angles, elevations or horns.

4. RutilarioideÆ.—Valves as if naviculoid, but with irregular or radial structure.

Groups 2 and 4 are not included in our description. No. 2 contains plankton genera only, while No. 4 consists of genera not yet found in this locality.