Clark — JJolophytic Plankton of Lakes Atitlan and Amatitlan. 99 



Order ZYGNEMACE.E. 



15. Spirogyra maxima (Hassall). 



Zygnema maxima Hassall, Annals of Nat. Hist. X, :!(>, 1842. 



Zygnema orbieulare Hassall, Brit. F. W. Alg. I, 138, pi. XIX, figs. 1, 2, 



1845. 

 Spirogyra maxima Wolle, F. W. Alg. U. S., 218, pi. CXXXIX, figs. 3, 4, 



188?! 



A large and exceedingly long Spirogyra, represented by one vial of 

 rather poorly preserved material, agrees very well with Wolle' s descrip- 

 tion. Pyrenoids prominent, making the spiral bands appear moniliform. 

 Diameter of the specimens at hand 120/*, length of cells 130/*. 



Not a plankton species properly speaking. Only the smaller species of 

 Spirogyra are oecasionaly found freely floating and are taken in the 

 plankton net. All the Spirogyras are, however, intimately associated 

 with plankton, as they form much of the food of rotifers and various 

 insect larva'. 



16. Spirogyra fluviatilis Hilse. 



Spirogyra fluviatilis Hilse, in Rabenhorst, Fl. Fur. Alg. 3; 243; 1868; 

 Wolle, F. W. Alg. U. S., 216, pi. CXXVI, fig. 1, 1887. 



Occurs in two bottles of rather fine filamentous alga? not belonging to 

 the plankton-haul series. The examples agree fairly well with Wolle' s 

 description, the chlorophyl bands are broader, and the pyrenoids promi- 

 nent, giving the bands a somewhat moniliform appearance. Bands 

 about 4, making about 2 turns. Diameter 36m, length of cells 7o/*. 



Order DESMIDIACE.E. 



17. Cosmariun retusum Perty. 



Cosmarium retusum Perty, Klein. Lebensf. , 208, pi. 16, f. 12 a-d, 1852; 

 Wolle, Desm. U. S., 80, pi. XVIII, figs. 25, 26, 1884. 



Not common; a few specimens occurring sporadically in different 

 samples of plankton from Lake Amatitlan. Our specimens agree well 

 with Wolle's figures. One measured 28/* long and 25/* wide at the suture. 

 Wolle gives " diam. of cells about 22/*." 



18. Cosmarium subcrenatum Hantzsch. 

 Cosmarium subcrenatum Hantzsch, in Rabenht., Fl. Eur. Alg. 3; 164, 

 1868; Wolle, Desm. U. S., 84, pi. XVIII, figs, <>, 7; pi. XIX, fig. 20, 

 1884. 



Not abundant, but generally scattered through the plankton samples, 

 one or two individuals occurring on quite a number of the slides from 

 different gatherings. The number of crenulations on the margin do 

 not agree exactly with those mentioned in descriptions, but the examples 

 bear a very close resemblance to published figures. An average specimen 

 measured 24/* long and 22/* wide at the suture. 



