. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . tains silica although it is not brittle. The amount ofsilica must, however, be very small ; less, even, than in some Desmids1. In Pseudotetraedron (vide Pascher, 13 A) and in Centritractus the cell-wallconsists of two halves, one of which slightly overlaps the other. It would also appear from Bohlins accountof specimens from the Azores thatthe wall of the cylindrical cysts ofCldorobotrys regularis consists of twohalves, but in the vegetative
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . tains silica although it is not brittle. The amount ofsilica must, however, be very small ; less, even, than in some Desmids1. In Pseudotetraedron (vide Pascher, 13 A) and in Centritractus the cell-wallconsists of two halves, one of which slightly overlaps the other. It would also appear from Bohlins accountof specimens from the Azores thatthe wall of the cylindrical cysts ofCldorobotrys regularis consists of twohalves, but in the vegetative cells ofCldorobotrys the wall is cells are globose except inPseudotetraedron and Centritractus ;in the first-named genus they arerectangular when seen from the frontand compressed when seen from theside or end. In this genus the cellsare also furnished with four longbristles, one at each angle of the Fig. 259. Botrydiopsis arrhiza Borzi. A, vege- front vjew_ Jn Centritractus thetative cell; B, formation of aplanospores ; •,•-,••, C, formation of zoogonidia. All x 600 (after cells are cylindrical with somewhatzi, from Wille).. swollen conical extremities, each ofwhich is furnished with a long bristle. There are two to many parietal 1 Chlorobotnjs is very abundant in the bogs of tbe British Islands (from which situations itwas first described as Chlorococcum regulare W. West) and the present author has found thatboiling in fuming nitric acid dissolves the walls more easily than in some species of Pleurotseniumand Euastrum. OphiocytiacesB 409
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