DESMID FLORA OF A TRIASSIC DISTRICT. 139 



for Dartmoor on the other) is quite equal to the far older geological 

 districts indicated by Professor West. The census list for Dart- 

 moor now stands at 290 species, 92 varieties, and about 30 forms, 

 which figures should be compared with 236 species and 68 varieties 

 given as the total Desmid flora of the British phyto-plankton 

 (9). It is obvious, therefore, that Dartmoor with its Post- 

 Carboniferous granite is equally rich with the Desmid areas on 

 the Lewisian granite. A consideration of this result suggested \ 

 the systematic examination of the Desmid flora on beds of a 

 still more recent geological period in order that some idea might 

 be obtained of how far that flora was influenced by the character 

 of the beds upon which its habitats occurred. 



The methods of collecting were the same as those adopted 

 for the Dartmoor district (10) ; the total time expended on the 

 field work, and the number of gatherings made, was roughly 

 also the same as for Dartmoor. As a rough guide to the richness 

 of the gatherings a " spread " under an inch cover glass was 

 prepared and the total of species, varieties and forms noted. 

 From the Dartmoor bogs a gathering that gave a total of sixty 

 by such a method was regarded as of maximum richness, and 

 similar totals were obtained from the better bogs in the present 

 district. With regard to the individual richness of the bogs 

 in the two districts some difficulty was experienced in arriving 

 at comparable results owing to the widely different character 

 of their formations. The bogs collected from on Dartmoor are 

 clearly marked areas due to depressions in the granite floor, 

 whereas the bogs in the present district being on an inclined plane 

 have no definitely outlined boundaries, but often succeed one 

 another over considerable distances. Where it was found pos- 

 sible to work out a fairiy isolated bog the numerical results 

 were surprisingly close to those of the Dartmoor bogs —that is to 

 say, the less important yielded an average of from 80 to 90 

 records, while up to 150 might be obtained from more extensive 

 localities. The material was collected during the months May 

 to October in the years 1917-1918, no gatheringworthy of mention 

 being made in the year 1919. The average rainfall for the dis- 

 trict in the years 1916-17-18 was 35*27 inches and the mean 

 temperature, 49'5 degrees ; the average rainfall being about 

 50 per cent, less than Dartmoor. 



The comparative table here given of the Desmid floras of the 

 JouRN. Q. M. C, Series II.— No. 86. 10 



