174 TUFTS COLLEGE STUDIES, VOL. II, No. 3 



and one pyrenoid ; repeatedly dividing by twos and fours, the 

 last generation being at the apices of dichotomous, gelatinous 

 strands, radiating from the position of the original cell, and be- 

 ing part of the cell wall left behind in the course of the daughter 

 cells outward ; the whole enclosed in a general gelatinous mass; 

 asexual reproduction by the transformation of a cell into a bicil- 

 iate zoospore, germinating immediately, or by its division into 

 2-4 daughter cells, giving rise to a new colony. Widely dis- 

 tributed plankton algae. 



KEY TO THE SPECIES OF DICTYOSPHAERIUM. 



i. Cells ovoid or ellipsoid. i. D. Ehrenbergianum. 



i. Cells spherical. 2. D. pulchellum. 



i. Cells reniform or cordate. 3. D. reniforme. 



1. D. EHRENBERGIANUM Nageli, 1848, p. 73, PI. II. K; 

 Wolle, 1887, p. 186, PL CL/VI, figs. 29-31 ; Massee, 1891, p. 

 457, PI. XII. Cells ovoid or ellipsoid, 6-10X4-7 M. membrane 

 thin ; colonies ovoid or globose, of 16-64 cells, up to 80 p, diam. 

 Fig. 59. N. J. Europe. 



The first division of the cell is into four ; all subsequent divi- 

 sions into two, each division in a plane at right angles to the 

 plane of the previous division ; reproduction by biciliate zo- 

 ospores, each formed from the contents of a cell, germinating 

 immediately. 



2. D. PULCHELLUM Wood, 1872, p. 84, PL X, fig. 4 ; Senn, 

 1899, p. 74, PL III, figs. 1-12; P. B.-A., No. 1511. Cells 

 spherical, 5-9 yu, diam., colonies up to 65 p. diam.; gelatinous 

 coating ample, with distinctly radiate structure ; reproduction 

 not observed. Me., Mass., Pa. Europe. 



3. D. RENIFORME Bulnheim, 1859, p. 22, PL II, fig. 6 ; 

 Wolle, 1887, p. 186, PL CLVI, fig. 28 ; Rabenhorst, Algen, 

 No. 789. Cells reniform or subcordate, usually 10-20X6-10 /A ; 

 colonies to 70 /w diam., somewhat irregular in form ; gelatinous 

 coating often with ciliate projections. Me., Mass., N. J., Pa. 



Europe. 



20. DIMORPHOCOCCUS A. Brauu, 1855, p. 44. 



Cells on gelatinous strands radiating from the center, at the 

 end of each strand 2-8 cells ; all cells similar, or differing in 

 the same group ; chromatophore occupying only the middle 

 part of the cell, with one pyrenoid. 



Free floating plants, resembling Dictyosphacrinm ; the develop- 

 ment not well understood. 



D. CORDATUS Wolle, 1887, p. 199, PL CLX, figs. 30-38; 



