The conjugation occurs in a similar manner, but the protoplasm of the cells divides transversely before conjugation into two daughter-cells. Those lying opposite one another conjugate (Fig. 16) and form two zygotes. The formation of the perizonium, and germination take place as in the preceding instance (_e.g. Epithemia_). 3. Two cells place themselves parallel to each other, and each of the two cell-contents, without coalescing, becomes an auxospore. The formation of the wall takes place as in the preceding case. This is found in the Naviculeæ, Cymbelleæ, the Gomphonemeæ (_e.g. Frustulia_, _Cocconema_). [Illustration: FIG. 16.--Conjugation of _Cymbella variabilis_. _A_, The protoplasm in the two cells has divided into two masses; _B_ these masses coalesce in pairs; the cells (_B C_) enclosed in a mucilaginous matrix. _C D_ Auxospores and their formation.] The Diatomaceæ may be found in salt as well as in fresh water (often in such masses that the colour of the water or mud becomes yellow or brown; in the same manner the genera _Chætoceros_, _Rhizosolenia_, _Coscinodiscus_, and several others, form large slime-masses, “Plankton” on the surface of the sea), on damp soil and in dust blown by the wind. They occur as fossils in the recent formations, often in large deposits (siliceous earth, mountain meal), as in the cement lime in Jutland, the alluvial deposits beneath Berlin, in clay strata beneath peat bogs, in guano, etc. These accumulations of fossilized diatoms are used in the manufacture of dynamite and in various manufactures. The Diatomaceæ appear nearest to, and must be placed as a group co-ordinate with the Dinoflagellata, as they doubtless may be supposed to derive their origin from forms resembling _Exuviella_, and to have lost the cilia. The resemblances to the Desmidiaceæ which are striking in many respects, can only be conceived as analogies, and cannot be founded upon homologies, and it is therefore impossible to regard them as proof of genetic relationship. The family contains only one order. [Illustration: FIG. 17.--Various Diatomeæ. _A Synedra radians._ _B Epithemia turgida_ (from the two different sides). _C Cymbella cuspidata. D Cocconeis pediculus_ (on the right several situated on a portion of a plant, on the left a single one more highly magnified).] Order 1. =Diatomaceæ.= This order may be divided into two sub-orders, viz.-- Sub-Order 1. =Placochromaticæ.= The chromatophores are discoid, large, 1 or 2 in each cell; the structure of the valves is bilateral and always without reticulate markings. The following groups belong to this sub-order: _Gomphonemeæ_, _Cymbelleæ_, _Amphoreæ_, _Achnantheæ_, _Cocconeideæ_, _Naviculeæ_, _Amphipleureæ_, _Plagiotropideæ_, _Amphitropideæ_, _Nitzchieæ_, _Surirayeæ_, and _Eunotieæ_.