Desmidiacece 



169 



median constriction is invariably deep. The semicells may be 

 elliptical, trapeziform, hexagonal or octagonal in outline, and the 

 angles are furnished with simple or furcate spines. The presence 

 of these spines constitutes the primary distinction between Xanthi- 

 dium and Cosmarium, and they are usually arranged in one plane 

 or in two parallel planes. In the centre of each semicell is a 

 thickened, scrobiculate area, or a protuberance of variable size, 

 and this character is the sole distinguishing feature between 

 Xanthidium and Arthrodesmus. In a few species the spines are 



Fig. 63. A, Xanthidium armatum (Breb.) liabenh., from Sligachan, Skye ( x 365). 

 B, zygospore of X. antilopaum (Breb.) Kiitz., from Ballynahinch, Galway, 

 Ireland ( x 365). 



reduced to small apiculations at the angles. The chloroplasts are 

 parietal in most of the species and are arranged in the form of 

 cushions, four or six in each semicell, each with a pyrenoid. In 

 some of the minute species there is a single, central chloroplast in 

 each semicell, furnished with one pyrenoid. 



The zygospores are globose and adorned with blunt papillae or 

 long slender spines, simple or furcate at their extremities. 



The genus was divided by Lundell in 187 1 1 into two sub-genera; 

 sub-genus Holocanthum, in which the spines are entire ; sub-genus 

 Schizacanthum, in which the spines are forked at the apex. These 



1 Lundell in Nova Acta Keg. Soc. Scient. Upsala, 1871, ser. 3, vol. viii, p. 74. 



