138 G. T. HARRIS ON THE 



tliis appear Cretaceous sands and sandstones, these beds resting 

 on the Keuper marls between Sidmouth and Seaton. Westwards 

 of the Otter Valley the district is mainly occupied by the Bud- 

 leigh Salterton Pebble bed which forms Woodbury Common, 

 and which also rests on the Keuper Marl. The numerous springs 

 that break out on the hill-sides at the junction of the Keuper Marl, 

 with the overlying beds give rise to small bogs of varying dimen- 

 sions, and it is in these bogs that the principal collecting has 

 been done. 



While the present paper may be taken as a further contribu- 

 tion to the Desmid Flora of Devon, the main object in view 

 when projecting the investigation was to compare the result 

 with that derived from an examination of the Desmid Flora of 

 Dartmoor (7) in order that comparison might elucidate in some 

 measure the influence of geological beds on the species density 

 of the Desmid Flora ; that is to say, to ascertain if the Desmid 

 flora of comparatively recent geological beds would stand com- 

 parison with such a proved rich Desmid area as Dartmoor, a 

 Palaeozoic moorland district, with ancient and extensive bogs, 

 mountainous altitudes (up to 2,039 feet) and excessive rainfall 

 (up to 80 inches). The late Professor G. S. West insisted that 

 only on the older Palaeozoic beds, and on the Pre-Cambrian beds, 

 could rich Desmid areas be found. He distinctly states that 

 " The rich Desmid areas correspond geographically with the 

 Pre-Cambrian and older Palaeozoic out-crops " (9) ; that, " In 

 the British Isles, the really rich Desmid floras are only to be found 

 in those areas which combine the most suitable habitats with 

 a drainage water derived from geological formations older than 

 the Carboniferous " (9). What Professor West understood 

 by a rich Desmid area may be gathered from his own writings : 

 " We do not apply the term ' rich ' to a mere abundance of 

 Desmids, or even to the occurrence of a great quantity of thirty 

 or forty species, but only to those areas in which 150-200 (or 

 even 300) species can be found in more or less abundance " (9). 

 Dartmoor by no means fulfils the conditions formulated by 

 Professor West for an ideal Desmid area, nor does it bear com- 

 parison geologically with the Welsh Lake District, or the Outer 

 Hebrides with their ancient beds of Lewisian granite, yet the 

 Desmid flora of Dartmoor (if one may judge from the published 

 lists of Professor West for Scotland on the one hand and of myself 



