170 



were put forward as genera by Wille 1 , but upon insufficient 

 grounds. Even in the most typical species which would fall under 

 Schizacanthum, namely X. armatum, the spines are sometimes 

 simple 2 , and in some of the tropical species there are numerous 

 intermediate stages between simple and much-forked spines. 



There are 14 British species, of which a few are general but not abundant, 

 and the rest are very scarce. X. armatum (Breb.) Rabenh. is a large and 

 handsome Desmid occurring in moderate quantity in the bogs of the hilly 

 districts of the British Isles ; length with spines 137 200 p, ; breadth with 

 spines 88 110 p.; fig. 63 A. X. antilopaeum (Breb.) Kiitz. (fig. 63 B) and 

 X. cristatum Bre'b. are not uncommon in certain areas, and X. continuum Arch., 

 which is the smallest species of the genus (length 9 9'5 p. ; breadth without 

 the apiculations 9'5 10'5/x), sometimes occurs abundantly amongst Sphagnum. 

 Species of this genus are regular constituents of the freshwater plankton. 



Genus Arthrodesmus Ehrenb., 1838. This genus is primarily 

 distinguished from Xanthidium by the absence of the protu- 

 berance or scrobiculated 

 area in the centre of the 

 semicells. As a general 

 rule the spines are fewer 

 in number than in Xan- 

 thidium, and they are all 

 disposed in one plane. 

 The median constriction 

 is deep and the semi- 

 cells, which may be 

 elliptical, trapeziform, 

 or subhexagonal in 

 shape, are furnished at 

 the lateral angles with 

 one or two spines of 

 variable size and 

 strength. In the verti- 

 cal view the cells are 

 always elliptical and the 

 poles are furnished with 

 spines. There is a single 

 central chloroplast in 

 each semicell contain- 



' J 



Fig. 64. A C, Arthrodesmus Incus (Bre'b.) 

 Hass., from Harrop Tarn, Cumberland; C, zygo- 

 spore (x365). D, A. Incus var. Ralfsii W. & 

 G. S. West, from Capel Curig, N. Wales ( x 365). 

 E, A. Incus var. validus W. & G. S. West (a form), 

 from near Glen ties, Donegal, Ireland ( x 474). 

 F and G, A. octocornis Ehrenb.; F, from Llyn 

 Idwal, N. Wales; G, zygospore from Puttenham 

 Common, Surrey ( x474). H J, A. bifidus Breb. 

 var. truncatus West; H, from Strensall Common, 

 N. Yorks.; I and J, from Keston Common, Kent 

 ( x 474). /, front view ; v, vertical view. 



1 Wille in Engler & Prantl's Natiirl. Pflanzenfam. 1890, p. 11. 



2 West in Journ. Linn. Soc. Bot. xxix, 1892, p. 164, t. xxii, f. 1. 



