66 BRITISH DESMIDIACE.ft. 



ference for the boggy margins of large pools and lakes, and 

 both the typical form and several varieties are regular con- 

 stituents of the lake-plankton. 



The hexagonal semicells with four pairs of spines are so 

 characteristic that this species in its typical form is not 

 easily confounded with any other. The scrobicnlations of the 

 central area are exceedinglv variable as can be seen by a 

 scrutiny of figs. 7, 8, 10-18, on PI. CVIII. 



Within certain limits the shape of the semicells is variable, 

 causing a corresponding variation in the length of the sinus 

 (compare figs. 7 and 8 011 PL CVIII). The spines are 

 somewhat variable both in length and acuteness, and they 

 may be straight or slightly curved, or even somewhat 

 recurved. 



The form with the straight and less acute spines has been 

 named " var. fasciculoides" by Lutkemiiller, but we hesitate 

 to separate this form as a distinct variety as the spines are 

 so variable in length and relative curvature, and the zygospores 

 of the straight-spined and curved-spined forms are exactly 

 alike. 



Borgesen, and also Larsen, have questioned the distinction 

 between X. antilop&um and X.fasciculatum, but judging from 

 the remarks made by Larsen in his 'Freshw. Alg. E. 

 Greenland/ 1904, p. 101, that author does not realize the 

 characters of X.fasciculatum. The form he figures is one of 

 the deformed specimens of X. antilopseum which are not un- 

 common in cold northern latitudes and in the plankton of 

 cold lakes. 



We have recognized six distinct varieties of X. antilopseum 

 in the British Islands, but the two characteristic American 

 varieties, var. minneapoliense Wolle and var. canadense Joshua, 

 in which a large spine occurs on, or in relation to, the central 

 protuberance, are not known to occur in Britain. 



X. antilopzeum var. incertum Schmidle ('Alg. aus Xyassa- 

 See/ 1903, p. 71, t. 2, f. 5) is not correctly placed under 

 X. antilopseum) and should be relegated elsewhere. 



Var. triquetrum Lund. (PL CIX, fig. 1.) 



X. antilop&iwn var. triquetrum Lund. Desm. Siiec. 1871, p. 76, t. 5, f. 1 ; 



Wolle, Desm. U. S. 1884, p. 94, t, 22, f. 1-3 [fio-ures poor] ; De Toni, 



Syll. Alar. 1889, p. 921 ; Borge, Chlor. Xorska Finmark. 1892, p. 8 ; W. 



& G. S. West, Phytoplankton Eiig-1. Lake District, 1909, p. 138. 

 X. antilopxum var. fasciculoides Liitkem. forma triqvetra Liitkem. Desm. 



Attersees, 1893, p. 547. 



A large variety ; vertical view triangular, with 



