204 BRITISH DESMIDIACE^. 



allied genera ; cell-wall smooth ; chloroplasts axile 

 with a central pyrenoid in each semicell ; cells embedded 

 in the ultimate ends of a radiating and branched system 

 of gelatinous strands, the latter being encrusted with 

 lime, aggregated in colonies 1-2 mm. in diameter. 



Zygospore angular, w r ith several mamillate projec- 

 tions. 



This genus is closely related to Cosmarium and Cosmodaditim. 

 From the latter genus it is distinguished primarily by the fact 

 that its cells are produced only at the periphery of the gelatinous 

 and usually lime- encrusted macroscopic colony. The more 

 asymmetrical form of the cells in Oocardium is of no great impor- 

 tance, since it has been shown by Senn that this is an effect of 

 the deposition of lime, and that if cultures are kept in lime-free 

 water, so that lime cannot be deposited, and the colonies simply 

 remain gelatinous, the cells become more symmetrical. 



The genus has been alternately placed in the Palmellacese 

 (Protococcales) or the Desmidiacese by various authors since it 

 was described by Nageli. There is no doubt, however, as to its 

 real affinities. The structure of its cell-wall and the nature of 

 its cell-division are sufficient to prove that it is a true Desmid. 

 The final proof of its systematic position has recently been 

 brought forward by the present writer, who was fortunate 

 enough to find the zygospores in some collections of algse from 

 India. There is only one known species of the genus. 



1. Oocardium stratum Nag. 

 (PI. CLIX, figs. 1-8 ; PL CLXVII, figs. 1-4.) 



Oocardium stratum Nag. Gatt. einz. Alg. 1849, p. 75, t. 3 A ; Kiitz. Spc. 

 Alg. 1849, p. 196 ; Tab. Phycolog. 1850, v. 2, t. 83, f. 5 ; Rabenh. Flor. 

 Eur. Alg. 1868, p. 53 ; De Toni, Syll. Alg. 1889, p. 658 ; Senn, Colonie- 

 bildende Alg. 1899, p. 81; Oocardium stratum, etc. 1899, p. 221; W. & 

 G. S. West, Alga-fl. Yorks. 1902, p. 130 ; Liitkem. Zellmem. Desm. 1902, 

 p. 360, t. 18, f. 37, 38, t. 19, f. 4-6 ; G. S. West, Brit. Freshw. Alg. 1904, 

 p. 174, f. 6(1, D-F ; Virieux, Alg. et Peridin. 1913, p. 4. 



Colonies small, 1-2 mm. in diameter, hemispherical 

 or longer column-like structures, gregarious, bright 

 green. Cells small, about \\ times broader than long- 

 in the front view (i. e. that in which the largest face is 

 visible and the constriction horizontal), depressed, very 



