COSMAEIUM. 187 



17<>. Cosmarium vexatum West. 

 (PL XCII, fig. 4.) 



Cosmarium ve;>.,if ,< / "\Vest, Alg-. Enoi. Lake District, 1892, p. 727, t. 9, 

 f . 33 ; Nordst. Index Desm. 1896, p. 270. 



Cells under medium size, a little longer than broad, 

 very deeply constricted, sinus narrowly linear "with a 

 dilated extremity ; semicells pyramidate-truncate, basal 

 and upper angles obtuse, sides convex and undulate, 

 with 6-7 undulations gradually increasing in size from 

 the basal angle to the upper angle, apex truncate, 

 straight or very slightly subundulate, within the 

 margin granulate, granules rather sparse and sub- 

 concentricaliv arranged (sometimes with a verv in- 



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definite radial arrangement), gradually diminishing in 



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size towards the smooth central area. Side view of 

 semicell ovate-circular or subcircular. Vertical view 

 oblong-elliptic, poles granulate, with a wide and smooth 

 tumour at the middle on each side, ratio of axes 

 about 1:2. Chloroplasts axile, with two pyrenoids. 



Zygospore unknown. 



Length 41-4o p. ; breadth 36-38 p ; breadth of isth- 

 mus 13'5-14ju; crass. 20-21 ,. 



ENGLAND.- -Ambleside, Westmoreland ! 



This Desmid occurred in considerable abundance among 

 various Alo'a? in a horse-trough, and also in an adiacent 



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ditch, at Ambleside. It stands nearest to C. Quasillus Lund., 

 especially in the gradual increase in size of the lateral undula- 

 tions from base to apex, and as in that species the apex pro- 

 jects, but in a much less marked degree. C. vexatum differs,, 

 however, in its smaller size, in the form of its semicells. in 

 the smooth and less prominent central inflation,, and in the 

 general form of its vertical view. 



Schmidle has described a " var. concavum" of this species 

 from Germany ('Alg. Geb. Oberrheins/ 1893, p. 550, t. 28, 

 f. 21), but apart from its somewhat larger size it does not 

 appear to differ in any essential point from the British form. 

 He describes the sides as concave, but figures them convex. 

 There is a very slight hollow on each side below the 



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apex in all the British specimens, thus causing the apex to 

 project very slightly. 



