254 G. T. HARRIS ON THE 



One of the most prolific gatherings was made at Postbridge the 

 second week in September from a small boggy ditch through 

 which a stream of water slowly passed. The gathering was 

 packed with zygospores of Staurastrum punctulatum Breb., 

 both type and its variety Kjellmanni Wille. Other genera and 

 species were represented but not to the same extent. Professor 

 G. S. West has stated that the end of September and beginning 

 of October is the period most favourable to the production of 

 zygospores, and the frequency with which they occur in the 

 Postbridge gatherings confirms this statement. As the majority 

 of them have already been described and figured I have made 

 no specific mention of them, with the exceptions of the zygo- 

 spores of Cos7narium Brebissonii and Roya obtusa var. montana 

 West. These two zygospores have not been figured, though as 

 West in the monograph of The British Desmidiaceae gives a 

 reference to Archer (10) concerning the zygospore of Cosmarium 

 Brebissonii I infer that some description of this zygospore is 

 there given. It occurred in Haytor gatherings collected at the 

 end of June, and several have been noticed. As will be seen from 

 the figure (PI. 19, fig. 10), it is globose and furnished with very 

 short conical projections, and measures about 90 fx in diameter. 

 The zygospore of Roya obtusa var. mow^awa West (PI. 19, fig. 11) 

 is ellipsoid and smooth, with a measurement of 22 yu, x 15 /x. 

 Kirchner states that the zygospore of the type {Roya obtusa) is 

 globose and smooth. 



W. and G. S. West have assigned a numerical standard indica- 

 tive of the relative richness of desmid areas (11), and judged by 

 it Dartmoor, with 280 species (certainly 300 when all are named) 

 and about ninety varieties, must rank as one of the richest desmid 

 areas of the British Isles. It is interesting in this connection 

 to refer to some records from other areas. The plankton of the 

 Irish lakes has a desmid census of 120 ; the Welsh lakes one of 

 113 ; the Scotch lakes one of 223 ; while the total Desmidiaceae 

 of the British phyto-plankton is given as 236 species and 68 

 varieties (11). In the Dartmoor desmid-flora are to be found 

 the following representatives of what W. and G. S. West con- 

 sider western types of British Desmids : Tetmemorus Brebissonii 

 var. minor ; Euastrum fictum (forma) ; E. validum ; Cosmarium 



