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



chromatophore and one pyrenoid ; asexual reproduction by suc- 

 cessive division into 2-32 cells, escaping by the splitting of the 

 mother cell wall into halves, still attached in part ; cells remain- 

 ing separate or uniting into a colony, spherical or sub-spheri- 

 cal, solid or hollow, joined by the gelatinous outer coating of 

 the cell wall. Widely distributed fresh water plankton algae ; 

 here arranged chiefly according to Senn, 1899, which is a care- 

 ful revision of the genus. 



KEY TO THE SPECIES OF COELASTRUM. 

 i. Cells united by arm-like processes from the membrane. 



5. C. reticii lain in. 



I. Cells in contact or united by quite short processes. 2. 



2. Cells with a short, free, external projection. 3. 



2. Cells without external projection. 4. 



3. Interspaces about equal to diam. of cell. 3. C. proboscideuin. 



3. Interspaces much smaller than diam. of cell. 4. C. cambricitm. 



4. Cells spherical or slightly elongate ; interspaces very small. 



I. C. microporum. 

 4. Cells ovoid, mutually compressed; interspaces \-\ cell diam. 



2. C. sphaericum. 



1. C. MICROPORUM Nageli in A. Braun, 1855, p. 70; Senn, 

 1899, p. 53. PI. II, figs. 11-17 J -P- B.-A., No. 1423. Cells 6-16 

 /A diam., spherical or slightly elongate and laterally compressed, 

 united by the gelatinous surfaces ; interspaces much smaller 

 than the cell diam. ; colony 40-55 /* diam. Mass., Pa., Porto 

 Rico. Europe. 



2. C. SPHAERICUM Nageli, 1848, p. 98, PI. V.C., fig. i ; 

 Wittr., Nordst. and L,agerh., Alg. Exsicc., No. 1241. Cells 

 4-20 p. diam., ovoid, mutually much flattened, deformed mostly 

 at the outer pole ; intervals between the cells about equal to 

 half the cell diam. ; colony 20-90 p. diam. Cuba. Europe. 



3. C. PROBOSCIDEUM Bohlin, i897a, p. 33, PI. II, figs. 19-22 ; 

 Senn, 1899, p. 59, PI. II, figs. 18-22 ; Wittr., Nordst. and 

 Lagerh., Alg. Exsicc., No. 1240; C. microporum Wolle, 1887, 

 p. 170, PL CLVI, figs. 1-3, not of Nag. Cells 7-40 /u. diam., 

 varying in form, but usually more or less angular, and with the 

 outer pole prolonged in some form, and crowned with a gelat- 

 inous thickening, varying much in size and shape ; colonies 

 usually of 2-16 cells in a loose network with relatively large 

 open spaces. Greenland, Me., Mass., N. J. 



Europe, So. America. 



C. microporum var. speciosum Wolle, 1887, p. 170, PI. ClyVI, 

 fig. 4, with projections like C. proboscideum , but with cells 



