256 G. T. HARRIS ON THE 



It will be seen from this table that several genera are very richly 

 represented in the district under review. Euastrum with 67 

 per cent, of the total British species is somewhat striking when 

 it is remembered that most of the species of this genus are among 

 the rarest of English desmids. All the species of the genus 

 Tetmemorus occur on the moor in profuse quantities, except 

 perhaps Tetmemorus minutus. The genera Spirotaenia and 

 Mesotaenium are certainly under-estimated owing to the diffi- 

 culty of correctly diagnosing species of these genera. On the 

 other hand the percentage of Cosmaria is very much lower than 

 one would have expected considering the large number of British 

 species. The genus Pleurotaenium is very poorly represented, 

 and Docidium not at all. In the genus Staurastrum the per- 

 centage given in the list is altogether too low for the number of 

 species actually occurring on the moor. 



In connection with the obvious richness of Dartmoor as a 

 desmid area it will be of interest to refer to a theory recently 

 advocated in explanation of these prolific areas, viz. that " the 

 rich desmid areas correspond geographically with the Pre- 

 Cambrian and older Palaeozoic outcrops" (11). With respect 

 to the age of the Dartmoor rocks Clayden states (12) that " it 

 must not be forgotten that the moor [i.e. Dartmoor] has been a 

 land surface ever since its superincumbent volcanic peaks first 

 arose above the waves of the Devonian sea." Accepting this 

 view of the geologists it will be seen that Dartmoor lends distinct 

 support to the theory that the richest desmid areas are connected 

 with areas older than the Carboniferous. The granite tors of 

 Dartmoor rise high above the region in which the bogs abound, 

 so that the drainage water of the bogs is largely derived from the 

 older Palaeozoic formation which West considers essential in the 

 constitution of a rich desmid area. 



While it cannot be said that any decidedly new species has 

 resulted from an examination of the Dartmoor bogs, several 

 " forms," and possibly some varieties, are of sufficient interest 

 to merit special mention. Professor G. S. West of Birmingham 

 University has been good enough to look over drawings of des- 

 mids from the material collected and to indicate those possessing 

 sufficient interest to make them worth reproducing. The figures 



