60 BRITISH DESMIDIACE.E. 



times with a pair of spines), all the spines upwardly 

 curved ; central area of semicell not differentiated or 

 very slightly thickened. Side view of semicell 

 circular. Vertical view elliptic. Cell-wall minutely 

 punctate. 



Zygospore unknown. 



Length without spines 36*5-38 /x, with spines 55- 

 59 /x, ; breadth without spines 31 /x, with spines 52-56 JK, ; 

 breadth of isthmus 7*6-8 p. ; thickness 20-21 /x. 



Geoyr. Distribution.- -United States. 



This is the only species of the genus in which the type 

 form is practically destitute of a differentiated central area. 

 As yet the typical form has not been observed in the British 

 Islands. 



In outline the semicells are very similar to those of 

 X. antilopseum, but it is a smaller species with the spines of 

 each semicell more upwardly directed, and often with only 

 solitary spines at the angles. It appears to stand in an 

 intermediate position between X. antilopseum and X. inchoatum 

 Nordst. 



Var. planctonicum W. & G. S. West. (PI. CVII, 



figs. 7, 8.) 



X. r-oiitroversum var. planctonicum W. & G. S. West, Scott. Fresliw. 

 Plankton, I. 1903, p. 539, t. 16, f. 2, 3; Further Contrib. Freshw. 

 Plankton Scott. Lochs, 1905, p. 72 ; Brit. Freshw. Phytoplaiikton, etc., 

 1909, p. 180. 



A larger variety, with the sinus less open and 

 linear with a dilated extremity ; central area of semi- 

 cells finely scrobiculated and with a small central 

 protuberance. Vertical view with a small tumour at 

 the middle on each side. 



Length without spines 49-52 ju,; breadth without 

 spines 46-49 /x ; length of spines 16-2o/x; breadth of 

 isthmus 12'5-lo'5/i; thickness 31 p.. 



SCOTLAND.- -Plankton of Loch Shin and Loch a 

 Clharbh Bhaid Mhoir, Sutherland ! Plankton of Loch 

 Grhriama, Sutherland (J. Mun-<ti/) ! Plankton of Lochs 

 an Sgath, Langabhat, Shrubhail, Stranabhat, and 

 an Tomain, Lewis, Outer Hebrides ! 



