"Full nature swarms with life; one wondrous mass Of animals, or atoms organized, Waiting the vital breath, when parent heaven Shall bid his spirit blow. The hoary fen, In putrid streams, emits the living cloud Of pestilence. Thro' subterranean cells Where searching sunbeams scarce can find a way, Earth animated heaves." 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. {13}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. DISCOIDEÆ 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. 1. COSCINODISCEÆ (_a_) _Meloseirinæ._--Frustules short, in chains. (_b_) _Coscinodiscinæ._--Frustules disc form, usually single, rarely in short chains. (_a_) MELOSEIRINÆ